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Necessary Tools for Solar PV installation

One, as a solar PV installer, requires several tools and equipments for the safe and successful installation. Solar PV systems are install...

Friday, 5 May 2017

Necessary Tools for Solar PV installation

One, as a solar PV installer, requires several tools and equipments for the safe and successful installation. Solar PV systems are installed on flat-roofs, pitched roofs and on ground. Each location has its own unique requirements and therefore needs specific tools. Yes of course, some tools are common.

Personal Protective Equipment (PPE):

Safety must be the foremost issue and hence the tools for personnel safety are of prime importance. The tools for personnel safety are helmet, safety belt or rope, hand gloves, boots, safety glasses, ladder etc. Each installer must have these tools, also called Personal Protective Equipment (PPE), and should be familiar with their use.

Tools for Array Layout Design:

Installation of Solar PV system starts with the array layout. The necessary tools required for array layout and installation are magnetic compass, measuring tape, chalk or marking pen, angle finder, torpedo level etc. Figure 1 shows a magnetic compass, measuring tape and marking pen.

Fig.1: A magnetic compass, measuring tape and marking pen.

A solar pathfinder is also needed for more precise design incorporating the shadow effects. It evaluates the solar energy potential at that particular location. A compass is not needed when you are using solar pathfinder. A careful and accurate layout helps to avoid any unwanted delay or shifting of solar array in future.

Installation of solar PV on pitched roof requires the exact location of roof rafters or trusses. That’s why installations on flat-roof are the easiest one, only the shade free surface area has to be kept in mind.

Hand-tools for structural support and module installation:

The hand tool required for installation of solar structure and modules are drill machine with assorted bits, cutter machine, spanner set, hammer, cutting plier, slip joint plier, nose plier, screw drivers of different sizes, utility knife, wire stripper, crimping tool, neon tester, hole punch etc. It’s better either to carry an extension board with you or all the power machines should be battery operated. Figure 2 shows a spanner set of assorted size, cutting plier, wire stripper, screw driver, neon tester, nose plier and hammer used for solar PV installation.

Fig. 2: A spanner set of assorted size, cutting plier, wire stripper, screw driver, neon tester, nose plier and hammer used for solar PV installation.

Ground mounted solar PV installations require pit digging for preparing the concrete base. Tools such as shovel etc are required. A spirit level and string line is required to install the solar PV modules in straight line and plumb.

After the correct installation of support structures, solar PV modules are placed and secured to the structure with the help of nuts and bolts. Usually spanners or sockets of two or three sizes, depending on the used nuts and bolts, are sufficient. Some installers use torque wrench also.

Tools and Equipments for Wiring:

Before wiring the PV modules, you have to check them for proper open circuit voltage. For this a digital multi-meter or digital clamp meter can be used. 

After ensuring that all the modules are OK and will properly work, they are connected in series, or parallel or series-parallel combination as per the approved or decided design.

For wiring of modules, sometimes lugs are required. Thus, one has to keep with himself a wire stripper and a crimping tool. Usually MC-4 connectors are used to interconnect PV modules, and for that you have to keep wire stripper and a crimping tool. Several crimping tool kits with interchangeable die sets are available in the market. 

After connecting all the modules, once again the voltage of the string or array should be checked for any in-correct connection. Proper checking at each stage will avoid troubles at a later stage. You may also check the PV module using a test load (may be a DC lamp capable of operating on the PV voltage). The current during the test can also be measured using the digital DC clamp meterFigure 3 shows a digital multi-meter and DC clamp meter used for PV module connections.

Fig.3: A digital multi-meter and DC clamp meter used for PV module connections.

Thursday, 4 May 2017


Efficiency of Solar PV Modules:

Photovoltaic (PV) technology uses semiconductor cells to absorb the solar irradiance and convert it into electrical energy. Commercially available solar PV modules currently have efficiency in the range 15 to 20%. Although the efficiency of the PV modules have increased over the years because of intense Research and Development efforts, there are many factors that lead to degradation of solar PV performance. As more and more MW range solar PV plants are coming up, there is an urgent need to pay attention towards issues that reduce the output of a solar PV module and hence the plant.

Factors affecting Solar PV output:

Among the various factors, the solar irradiance and cell temperature greatly affects the output energy from a solar PV module. Low irradiance and higher cell temperature significantly reduce the PV output. Cell temperature is key issue particularly in countries like India. 

In addition to these factors, the output energy from a solar PV module or solar plant also depends on the reliability and performance of the Balance of System (BoS) and environmental factors. The various losses that occur in a PV system or plant are mismatch loss, wiring loss, inverter loss, sun-tracking loss, shading loss, soiling loss and solar dimming loss.

Soiling Loss in Solar PV Plant:

This article is focused mainly on soiling loss and solar dimming loss that do happen in a solar PV plant. 
"Soiling Loss is the loss in PV output power and hence energy resulting from accumulation of dust, dirt, snow and other particles such as tree leaves and bird’s dropping, etc. over the surface of the PV module." 
Dust is a thin layer that covers the surface of the PV panels and its size depends upon the location of the PV plant and its environment. Accumulation of dust may be due to wind, human activities, vehicular movements, industrial and constructional activities etc. The amount of accumulated dust over the PV module surface reduces the daily, monthly, and annual energy yield of the solar PV plant. Study shows that Middle East and North Africa have the worst dust accumulation areas in the world. India too is prone to heavy dust deposits, particularly the states like Rajasthan. 
In one of the study carried out in Baghdad in the year 2016 showed an 18.7% decline in the efficiency of solar PV modules, when left unattended for a month. A new study, carried out under the guidance of Prof. Bergin of Duke University in one of the IIT campus, has found that dust and particulate matter (PM) are reducing the energy output of solar PV plants in Northern part of India by 17-25% annually. 

The study,  found that half the reduction comes from dust and particles deposited on the surface of solar PV modules, which forms a physical barrier to sun-light entry. Other half of the reduction came from the atmospheric pollution that reduces the sunlight reaching the ground, also known as “solar dimming”. 
Solar dimming or Global Dimming is the gradual reduction in the amount of direct irradiance at the earth’s surface. The effect varies by location and is supposed to have been caused by an increase in particulates such as sulphate aerosols in the atmosphere due to human activities. 
According to a study carried out at the Indian Institute of Tropical Meteorology, Pune, the amount of Solar radiation reaching India’s land mass is reducing by 0.86 Watts/ square metre per year. 

The output of solar PV module or panel is very much dependent on the solar irradiance on the module surface. Or in other words, the output power of the solar PV module is directly proportional to the solar irradiance. The standard solar irradiance for calculating solar PV output and performance is 1000 W/m2. At half the standard irradiance, the output power of the solar PV module is also reduced to half, as can be seen from figure 1. Lower solar irradiance also reduces the output voltage of the PV panel. Figure 1 shows the power-voltage ( P-V) curve of solar PV module at different solar irradiance level  at constant temperature of 25 oC. 

Now a days several software are available for assisting the solar PV designing which can very much predict and asses the PV module behaviour under varrying solar irradiance and other variables.
Fig1: P-V curve of solar PV module at different solar irradiance level ( at 25 oC)

A similar study carried out in Singapore found that the solar PV output has been reduced by 15-25% because of poor air quality. In the worst case the solar irradiance was recorded as low as 50%. 

This means that cleaning of solar PV panels is not enough to ensure maximum energy from a solar PV plant; environmental quality and pollution level also plays a significant role. In future solar PV plant developers will be extra cautious when signing Power Purchase Agreement (PPA) with clients having solar PV plants located in highly polluted areas.  

Saturday, 11 February 2017

Digital Clamp Meter: A more versatile Measuring Instrument

Measurement of Current..

Yeah! the usual meter that comes in mind for current measurement is the ammeter. These meters have to be connected in the circuit to get the reading. Thus, for current measurement using ammeter, we have to disturb the circuit and put the meter in between. Also the prior knowledge of the magnitude of circuit current and it’s nature is necessary. One should also know the proper method and have the means to disconnect and reconnect the circuit.

Clamp-On ammeter is a very handy device which can measure the current flowing through any LT circuit without disturbing the existing connections.

Clamp meter:

A clamp meter is a more versatile instrument that combines a clamp-on ammeter with a multi-meter. It is usually capable of measuring alternating current, AC/DC voltage, and resistance. Many of these meters are able to test the capacitors, measure frequency, temperature, continuity etc. Continuity is a quick test to check an open circuit. When the circuit is closed, the meter emits a “beep” sound. 

The display can be analog or digital; accordingly are called analog or digital clamp meters. Digital meters have several advantages over analog ones and thus are commonly used. Figure 1 shows a digital AC/DC clamp meter.

Fig.1: A digital AC/DC clamp meter

Clamp meters have a movable jaw that can be opened. Open jaw allows the clamp meter to be clamped around a current carrying wire to measure the current flowing through the circuit. Figure 2 shows the digital clamp meter with open jaws.

Fig.2: Digital clamp meter with open jaws.

Working of DC clamp meter:

Conventional clamp meters, capable of measuring alternating current only, do not show the reading when used to measure DC current. DC clamp meters work on the principle of “Hall Effect”. The magnetic field because of DC current produces a small voltage across the Hall Effect sensor. This voltage, which is proportional to the DC current flow, is further amplified for measurement.

DC clamp meters or AC/DC clamp meters are very useful in Solar PV system installation and maintenance, since the output current of a solar PV plant is DC in nature. 

Accessories such as pair of red and black probes for voltage measurement and thermocouple with leads (clamp meters with temperature measurement facility) are provided. Figure 3 shows the thermocople with leads. Figure 4 shows how the thermocouple leads are connected to the clamp meter (also shows the temperature).

Fig.3: Thermocople with leads

Fig.4: Thermocouple leads connected to the clamp meter

The digital AC/DC clamp meter is a battery operated (9V) device. When the battery is low, a “low- battery” indication appears on the display. The “Auto power off” feature automatically turns off the meter when no operation is detected for 15 minutes, thus helps to extend the battery life.   

Advance Features of Clamp Meter:

Several digital clamp meters offer a wide variety of advanced features which ultimately help an electrical engineer to a great extent. These features are:

1.  PC Interface – Clamp meters also come with PC interfacing serial ports to facilitate data transfer to computer for further analysis of data and report generation.
2. Data Logger – Some clamp meters have internal memory capable of holding the measured values to be used later on.
3. Inrush Current measurement – This feature allows the clamp meter to measure the high inrush current usually encountered during motor starting.
4. Autoranging – Automatically sets appropriate measurement range thus, avoids manual selection of range.

Safety Requirements and Standards:

The AC/DC clamp meters are designed accordingly to safety standard IEC61010-1 and IEC 61010-2-032 to meet the safety requirements for electrical measuring instruments and hand-held meters. They also comply the European standards 89/336/EEC for Electromagnetic Compatibility and 73/23/EEC for low voltage  


A few things should be followed so as to ensure that these meters last long in the harsh environment encountered in the field. The checks to be carried out are –
1.  Keep the meters in their safety cover or carry bag after the use to avoid ingress of moisture, dust etc, and shocks and damage.
2.  Do not store these meters in a strong magnetic field to avoid loss of accuracy. Electrical noise, unwanted signals or intense electromagnetic fields in the vicinity may disturb the measuring circuit.
3.  All meters are very fragile and hence should be handled with care.
4. Do not expose the meters to extreme hot or cold temperature and moist atmosphere.
5. Always select the proper range and nature of circuit i.e. AC or DC.
6. Always inspect the meter and test leads for any sign of damage or abnormality, such as broken or cracked, leads before use.

Friday, 3 February 2017

Ancillary services for Indian electric market

In a vertically integrated power system, one organization carried out all the work of electricity generation, transmission and distribution. Example is the erstwhile State Electricity Boards in India. Power industry throughout the world is undergoing the restructuring process and is adopting the deregulated structure for better utilization of resources and for providing quality services to its consumers at an economical price. 

The introduction of deregulation has brought in several new entities and some form of competitive electricity market. The vertical integrated utilities were separated into Generating companies (Gencos), Transmission Companies (Transco’s) and Distribution Companies (Discos). Apart from these three entities,  entities such as System Operators, Market Operators, Regulators etc were also formed. 

Central Electricity Regulatory Commission (CERC) Regulations 2010 defines Ancillary services as those services that are necessary to support the power system operation in maintaining power quality, reliability and security of the system or the grid. Examples are active power support for real time load following, reactive power support, black start, voltage control support etc.

In the deregulated environment, ancillary services are no longer treated as integral part of the electric supply. The system operators have to purchase these services from ancillary service providers. The CERC Regulation, 2010 made operation of ancillary services as an exclusive function of Regional Load Despatch Centre (RLDC). In the Central Advisory Committee meeting of CERC, held on 14th March 2012, there was a general consensus on the need for introducing ancillary services to enhance the secure and reliable operation of Indian power system.     

Main ancillary services under consideration in India:

Currently three main types of ancillary services are under consideration in India and they are:
1.      Real power support service or Frequency support ancillary service or Load following,
2.      Voltage or Reactive power support services, and
3.      Black Start support service.

Real power support service or Frequency support ancillary service or Load following:

In India, the real power support or the frequency support services are to be provided by generating station or any other authorized entity on behalf of the generating station. The services/capacity of the said station should be available for despatch as required by the nodal agency i.e. RLDC to support the system frequency. 

Considering the Indian power system as a whole there are cases when certain surplus energy, particularly from Captive power plants, lie un-utilized at some point of time at some location. At the same time some portion of the system is facing power shortage and carrying out load shedding to cope up with the situation. 

Therefore, there is an urgent need for service mechanism such as Frequency Support to properly utilize these unused capacities to enhance the Grid security. Initially generators having surplus generation, either because of un-requisitioned quantum by the beneficiaries or quantum of generation which could not find buyers or captive generation capacity, may bid for the frequency support when their services are sought upon by the system operator. 

Frequency support services can also help to mitigate the intermittent nature of renewable energy sources such as wind and solar farms. At this juncture it is worth mentioning that Indian government has set some very ambitious targets towards renewable energy generation particularly from wind and solar.     

Eligibility Criterion and Implementation
All the producers and the regional entities which are members of the scheduling and deviation settlement mechanism for real and reactive power are entitled to participate in the ancillary market, but the condition is that they must have relevant telemetry facilities. 

No other No Objection Certificate (NOC) or clearance, other than that issued by the respective RLDC/SLDC for participation in the Day-ahead Market (DAM), is required. The Frequency Support Ancillary Services (FSAS) are to be implemented through bidding in the power exchange. The participants are free to bid in any of the Power Exchanges of the country for ancillary services. The bids for FSAS to be dispatched on next day are to be opened up after the closure and clearance of the day-ahead market. Time-block-wise bids with the quantum, price and the location (where services are to be given) are to be submitted for the next day despatch.  

Reactive Power Ancillary Services

Unlike the frequency, voltage is a local phenomenon, and hence the requirement for reactive power support from capacitors or reactors at the different sub-stations or generating stations may vary. Thus, reactive power ancillary services are also in the list of ancillary services in India. Although there is a provision in the IEGC regulations and according to which proper voltage profile is to be maintained at all interchange points between the control areas in the Indian power grid.

However this is not sufficient and there is an urgent need for reactive power support to be added in the ancillary services list. Whenever there is persistent low voltage profile at one or more points in the system (this may be frequent during agricultural season), the system operator should have the voltage support ancillary tool in his armory. 

The voltage support ancillary service provider may also bid through the power exchange. Since the reactive power and hence the voltage support is a local phenomenon and their requirement are different at different locations, therefore mobile reactive power supports on trailers etc. can be very useful in the Indian power system. It is supposed that initially the government transmission companies will provide the mobile reactive power support and further opening up of the scenario will depend on their outcome.      

Black Start Ancillary Services

Black start is the process of restoring a power station to operation without relying on the external electric power from the transmission network. Normally the electric power required by a plant for its operation is provided by its own generators. If all the generators of a generating plant are out of service then to restore the generation, the station requires the electric supply externally, drawn from the transmission line connected to the station. In the event of power outage in a wide area, the transmission lines are also out of supply. In the absence of transmission line or grid, the so called Black Start facility is required.   

The Central Electricity Authority (CEA) Regulations 2007 mandates the Hydel power stations in the country to provide the Black Start facility. There is a provision of incentives to such service providers who offer their services (black start services) when asked by the system operator. Black Start Ancillary Services (BSAS) are to be paid when it is required by the SLDC i.e. the nodal agency. The generators capable of providing the start up power are to be paid for one day capacity charges for their services. The quantum of energy supplied during the restoration process is paid (energy prices are paid the BSAS provider) at twice the energy charges as determined by the Commission.  

Ref.: “Introduction of Ancillary services in Indian Electricity Market”, Central Electricity Regulatory Commission, New Delhi, April 2013

Monday, 30 January 2017

Role of Simulation Softwares in Solar PV plant Design and Operation

The performance of a Solar PV plant depends on the solar irradiation, the optimum tilt angle, ambient temperature, design parameters, quality of modules, efficiency of inverters etc. The output power of solar PV module is directly proportional to the solar irradiation.

Earlier the expected generation output, losses in cables & equipments, voltage drop, lower and upper range of array voltage etc. were manually calculated. Now-a-days this tendency is fast being replaced by advanced simulation software, particularly in the case of solar PV plants with significant capacity.

Function of Simulation Softwares in plant Design and Operation:

Softwares are used as essential tools in the design, construction, operation and maintenance of Solar PV plants. They help to produce solar PV generation assessments, and site-specific production forecast accounting the location latitude and longitude, weather condition, shadow effect etc. These softwares lend a hand to optimize design configurations and system layouts, test the system performance. 

Economic evaluation and payback period calculation are also facilitated by these softwares. They also make possible the integration of PV plant output to the Utility grid, monitoring and control of the plant health, thus enhancing the operation of the PV plant.

Popular softwares:

There are several softwares available for assisting the solar PV system designing. The popular are:

1.       PVSYST
2.       PVSOL
3.       Solar Advisor Model (SAM)
4.       SMA off-grid Configurator
5.       Solar Pro
6.       PV Design Pro
7.       HELIOS 3D

PVSYST is one of the oldest and trusted PV designing software developed by the University of Geneva. The software offers a wide range of features, a few are –

1.Complete designing of off-grid and grid connected solar PV system.
2.Complete data base of solar modules and inverters.
3.Meteorological data of a large number of stations and sites across the world. Data of new sites can also be imported.
4.Access to PVGIS and NASA database for import of irradiation data.
5.Enables economic evaluation and payback period.
6.Predicts and asses the module behaviour under varying irradiation, ambient temperature and shading scenarios.

PVSYST software has two modes:
1.       Preliminary mode, and
2.       Project design mode.

In the preliminary mode one can get the approximate value of power generation. Project design mode allows user defined values for inverter efficiency, losses and shading effect. The results of these softwares are in the form of report, tables and graphs. The software is available in a trial version with one month access period. Cost of the software depends on its version and the number of design to be carried out per month.     

 Free Softwares:

Some free to download softwares such as PVGIS Estimation Utility, SMA Sunny Design etc are also available which can be used in the design of solar PV system.

PVGIS Estimation Utility is one such software which is freely available. With the help of this software one can calculate the output of a proposed solar PV plant, optimize the design and can have a fair idea about how the PV system is going to perform when completed.

Another free software is “SMA Sunny Design” by renowned solar inverter manufacturer SMA. The software generates detailed reports in “pdf” format of the proposed solar PV plant. The typical input required are the number and type of PV module to be used, location details, roof pitch etc. The software is able to compare the performance of PV panels of different types, makes and models. Although the software has the database of most of the solar PV panels, but one can have the option of feeding his/her own PV module details.

How the Software help out in plant Operation:

After the successful commissioning of a Solar PV plant, the next important activities are monitoring and operation & maintenance of the plant. There are softwares that help in carrying out these works also. The software can indentify under-performing plant and arrays, and thus help in increasing the power production. Incoming data from the PV modules, instruments to monitor the ambient, inverters, transformers and grid interfacing equipments are analyzed. Smart and advanced sensors and data loggers collect this information that affect the PV plant’s performance, and allow the software to create real time performance models. Variances are detected, and notifications and alarm messages are given to the concerned agency/ person.

Thus these softwares can create custom analytics, alerts, contracts and reports. They also helps in streamlining the operation & maintenance activities, and hence reduces Mean Time to Repair (MTTR). They also help in forecasting the power generation of the PV plant based on the weather forecast.  

 One such software is the Qantum Solar Monitoring and O&M software.

Friday, 27 January 2017

Digital Multi-Meters: A very handy tool for Electrical Engineers

An electrical engineer and electrician must be familiar with a variety of test and measurement instruments. They need to know how to use these instruments in a safe and proper way under different working conditions. It is important to use the right tools and instruments when working with electricity. Appropriate equipments can make a hard task easy and at the same time wrong selection can make an easy job difficult and dangerous.

This article looks at the most common type of meter available with an electrical engineer or electrician; i.e. the Multi-meter. 

Another versatile and handy instrument for an electrical engineer, particularly in the Solar PV field, is the digital clamp meter

Multi-meters are hand-held instruments designed to measure more than one electrical value; for example DC and AC voltages, DC and AC current, resistance etc. They are also used to check the continuity of conductors or to check the diode. Figure 1 shows the icon of some of the electrical quantities measured by a multi-meter.

Fig.1: Icon of some of the electrical quantities measured by a multi-meter.

In other words, a multi-meter is a measuring instrument that combines an ammeter, a voltmeter and an ohmmeter to measure the current, voltage and resistance in a circuit; means no need to carry different meters for measuring different values.

Digital Multi-meter:

Both analog and digital multi-meters are available, but the digital version has superseded the analog one. Digital Multi-meters (DMM) has a digital display to show the measured values, a dial or selector switch to change the mode of the meter, and connecting terminals. Figure 2 shows a low cost commonly available DMM.

Fig. 2: Low cost commonly available DMM.

Accessories such as a pair of red and black probes, is also provided with the meters. The black probe is always considered as the negative probe and is connected to the common terminal. Figure 3 shows the probe provided with the DMM.

Fig. 3: Probe provided with the DMM.

System with Harmonics:

When harmonics is present in the system, only true RMS meters can provide accurate measurement   of AC values. Harmonics are frequencies that are multiples of the fundamental frequency (50 Hz in India) and can cause distortion of basic AC waveform. The common loads which causes harmonics in the system are computers, fluorescent luminaires with electronic ballast, inverters etc. These electrical loads are also called Non-linear loads as their impedance are not constant.

Working of Digital Multi-Meter:

The basic circuit of a Digital Multi-Meter is always a DC voltmeter. The AC quantity is rectified into DC with the help of rectifiers and filters. All the quantities are digitised using an Analog to Digital Converter (ADC) and displayed in the digital form on the display.

Figure 4 shows the internal circuit of a commonly available Digital Multi-Meter.
Fig.4: Internal circuit of a commonly available Digital Multi-Meter.

Advantages of Digital Multi-Meter:

Digital Multi-Meters have the following advantages over analog:
1.       Cheap and compact,
2.       Accuracy is very high,
3.       No loading effect,
4.       Reading of values is much easier,
5.       Can be interfaced with external equipment,


“Always read and follow the manufacturer’s instructions that are supplied with the Multi-meter. Equipments may be damaged badly and or the user may be seriously injured if proper safety procedures and instructions are not followed.”
Safety is very important when using test and measurement instruments. Many a times, the meter is used to make the measurement while the electrical circuit is energized. Ensure to follow the regular and standard safety procedures when working on live circuits. A safe working practice is always mandatory. The following safety tips are recommended-

1.       Ensure the correct position of selector switch,
2.       Do not be alone when working on live circuits,
3.       Keep your hands dry when taking measurements,

4.  Use rubber gloves and wear shoes when taking measurements on energized circuits.

Monday, 26 December 2016

About the Solar Charge Controller

A solar PV system has several components. Apart from the solar PV panels, it has charge controller, inverter, battery bank, supporting structure, protective fuses, breakers & surge protectors, cables etc. All the components other than solar PV panel are collectively called Balance of System (BoS). A well designed system is required for the smooth and reliable operation of the Solar PV plant.

What is Solar Charge Controller and how it works?

A Solar Charge Controllers (S.C.C.) or simply Charge Controller monitors and controls the power output from the solar PV panels and the battery. The Controller controls the flow of charge into the battery while charging from the solar PV system, thus prevents overcharging of batteries. It detects the status of battery charge by measuring the terminal voltage of the battery.

In overcharge condition, the battery terminal voltage increases above a certain level. When the battery reaches overcharge status, as shown by the measured voltage, the charge controller cuts off the solar PV supply to the battery.

As the connected load continues to use the battery energy, the terminal voltage of the battery drops down. This drop in voltage is also detected by the charge controller as it is continuously monitoring the activities. Whenever the battery voltage reaches the normal operating range, the battery is connected back to the solar PV system, by the charge controller, for regular charging.    

Similarly, the charge controller also controls the discharge from the battery. It prevents deep discharge of the battery. In deep discharge condition, the battery terminal voltage decreases below a certain level. This happens because of excessive drainage of charge from the battery, probably due to prolonged use of significant load. As the battery gets into deep discharge state, the charge controller detects it from the measured voltage and disconnects the battery from the circuit, so that no current can be further drawn from the battery. The life of a Lead-Acid battery very much depends on the Depth of Discharge (DoD).

"For example, as per Luminous, a renowned solar battery manufacturer in India, solar Flooded Lead-acid Tubular batteries have a life of 1500 cycles at 80% DoD, 3000 cycles at 50% DoD and 5000 cycles at 20% DoD."
Thus, both overcharging and deep discharging of battery or battery bank must be avoided to enhance the battery life.

Features available in Charge Controllers:

A Charge Controller usually/ may have the following features:

1.       LED or LCD display of various parameters or functions,
2. Automatic Temperature Compensation feature; automatically adjusts the battery charging according to the ambient temperature,
3.  User defined setting of battery voltage and type,
4.  Protection for low voltage, overvoltage and reverse connection,
5.  Automatic priority selection feature,
6.  Equalization feature.
Omission or addition of certain feature may happen, and it depends on the manufacturer.

Types of Charge Controllers:

The two commonly available types of Charge Controllers are:

     Pulse Width Modulation (PWM) Charge Controller, and
2.  Maximum Power Point Tracking (MPPT) Charge Controller.

Pulse Width Modulation (PWM) Charge Controller:

PWM Charge Controller is a solid state controller that usually works on three step charging algorithm. It has a semi-conductor switch which is switched ON and off by PWM at a variable frequency to maintain the battery voltage. When the battery voltage reaches the pre-specified value, the charging current is reduced as per the charging algorithm to avoid heating and gassing of the battery.

The PWM charge controller adjusts the charging according to the battery condition and requirement by controlling the speed of the switching element, which breaks the PV output current into pulses at some constant frequency and varies the width and time of pulses to regulate the amount of charge flowing into the battery.

Pulses of current helps the battery as it mixes the electrolyte, clears the lead electrode and prevents sulphation.

PWM charge controller maintains the battery capacities to 90-95% and has the ability to recover lost battery capacity. This helps in equalizing drifting of battery cells, automatically adjusts battery aging, increase the charge acceptance of the battery and self regulates the voltage drops and temperature effects of the solar PV system. These charge controllers are cheap and available in a wider range of capacities.

PWM charge controller is a good low cost option for small roof top solar PV systems where the ambient temperature is moderate or high.

Drawbacks of PWM charge controller:
1. The controller voltage must match the battery bank voltage, and
2. Usually the maximum current capacity of PWM charge controller is limited to 60 A.

Maximum Power Point Tracking (MPPT) controller:

Maximum Power Point Tracking (MPPT) controller is a charge controller with an additional device called the Maximum Power Point Tracker. These controllers provide a digital tracking of the PV panel output and compare it with the battery voltage. It then works out the maximum power that the panel can flush out to charge the battery or to the load. It tracks the optimum voltage so as to get the maximum amperage to charge the battery. Actually it is the amperage which makes sense for the battery.

MPPT charge controllers have higher efficiency, thus higher output power and overall better battery management than PWM charge controllers. These charge controllers continuously adjust the load on the solar PV system under varying operating conditions and keeps it operating at the maximum power point. As mentioned earlier, the controller checks the output of the PV array and compares to the battery voltage. It then calculates the maximum power that the PV array can produce. Accordingly, the controller converts the PV output voltage and converts it to the optimum value that fetches the maximum current into the battery or the load.

MPPT power varies according to the weather conditions, i.e. solar radiation, ambient temperature, and cell temperature. The voltage point at which the PV system produces maximum power is called Maximum Power Point (M.P.P.). Thus, the MPPT charge controller acts as a DC voltage converter which converts the PV array voltage into a voltage that fetches maximum power. The converter converts the DC input from the PV into a AC voltage and converts it back to DC matching the battery voltage. A Buck converter is used to step down the voltage, whereas a Boost converter is used to step up the voltage. They are used in PV systems of higher capacity, although MPPT charge controllers with smaller capacity say 17 A, is also available in the market.

For a PWM charge controller, the output current is the same as the input current, whereas for a MPPT charge controller, the output power is the difference of the input power and controller losses. With an MPPT charge controller employed, the system can deliver 20 to 45% more power in winter and 10 to 15% more in summer. The actual gain may vary depending on the weather temperature, state of the battery charge etc.

Figure 1 shows a 12 V, 17 A MPPT solar charge controller of Su-Kam make which is available in approximately 2,300 INR (in Bhopal, MP).

Fig.1: 12 V, 17 A MPPT solar charge controller of Su-Kam make 
MPPT charge controller has the following advantages:

1.  More effective in low temperature and cloudy days,
2. Have better efficiency in the range 93-97%. ( much better than PWM controller).
The drawbacks are complex circuit and comparatively higher cost.

In some solar PV systems, the charge controller and the inverter are collectively housed in a single unit and are called the Power Conditioning Unit (P.C.U.). Batteries are also among the key elements used in off-grid solar PV systems and hence one should know a bit about these storage devices.

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