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Solar PV System

Design Methodology for Standalone Solar PV System

1.0 INTRODUCTION

Sizing of Solar PV systems is not at all difficult if you know what appliances will be used and how long they will operate each day. Because all power must come from the solar panels, it is most important that they are large enough to provide the energy needed even on cloudy days. In this chapter, we will learn how to design a solar PV system to meet the energy requirement of a given household.

Each household has different set of appliances, their pattern of usage varies and hence the electricity requirement of each house is different. For a solar PV system to work properly, the size of the panels and the battery must be matched with the energy requirements of the appliances. Because panels and batteries are expensive, people often try to save money by installing too few panels or too small a battery. This is a very poor practice and does not really save money, because a system that is too small for the appliances does not work well and the battery will have to be replaced often at high cost. As a designer and installer of solar PV system, we should take care of varying requirements and then should try to design customized PV solutions to meet the requirement of our customer.

1.1 DESIGN METHODOLOGIES

The design of a solar PV system involves determining the capacity and size of various components to be used in the system to supply reliable electricity to the connected load as required. A lot of parameters like the location of PV system installation, amount of sunlight available there, temperature of that location, dust in the environment, loads to be connected and hours of usage of each load, seasonal variation in load, available components in market and their ratings etc. affect the performance of the PV system. All these parameters are considered while designing a solar PV system. Thus, the design of a solar PV system can be categorized in two ways:

  • Approximate design
  • Precise design

 

APPROXIMATE DESIGN

In Approximate design methodology, the design of PV system is made simple and approximate.  Only a few parameters (that affect the PV system performance as mentioned in the earlier paragraph) are considered and some simple assumptions are made in this design process. Some parameters like the effect of temperature and radiation, seasonal variation in load etc. are neglected in this case. This methodology is acceptable for small solar PV systems, domestic applications etc. which is within a few KWp range. In this case, the designed system will not be 100% correct, but it serves the purpose and should provide an accuracy of up to 80% to 90% of design.

PRECISE DESIGN

For designing larger PV systems of several 10s or 100s of KWp or higher, ‘precise design’ methodology is desirable. This methodology for designing precise PV systems is same as that of designing approximate PV systems with the exception that all the parameters that affect the performance of PV system are considered in the precise design process.

1.2 APPROXIMATE DESIGN OF OFF-GRID SPV SYSTEM

In the previous chapters, we have learned about various components that are used in solar PV system. In this section, we will mainly focus on the design of standalone SPV systems. A stand-alone (or off-grid) PV system allows you to run all your electricity from your PV system without using the utility power (from grid) or any other power supply.  In this system, if we need to power the load during non-sunshine hours, then we have to store the energy generated during day time using batteries. A standalone SPV system consists of Solar PV modules, batteries, charge controller, inverter and loads as shown in fig. below.

 

 
   

The energy generated by solar PV modules is stored in batteries and supplied to the load as required. The flow of energy is from PV module to battery and then to load as shown below.

 

During sunshine hours, energy generated in the PV modules flows to the battery through MPPT and charge controller in the electronic control circuitry.  The energy stored in the battery flow to the load through the charge controller when the load is operated and in case of AC loads it flows via inverter.

 

1.3 DESIGN PROCEDURE

A Standalone PV system design proceeds in the reverse direction to the flow of energy. That is, first we assess the power requirement of connected load and based on that, we design the electronic part namely Inverter and Charge Controller, then the Battery bank, PV module and finally wires, fuses & junction boxes. Thus, the procedure followed in the approximate design methodology of solar PV system is as follows:

Step1: Determine the demands of power consumption i.e., the connected load andtheir energy estimation.

Step2: Determine the Size and Choice of electronic circuitry like Inverter, MPPT Charge controller etc.

Step3: Determine the size of Battery bank.

Step4: Determine the size of PV module.

Step5: Determine the size of wires, fuse, Junction box etc.

 

 STEP1: LOAD (ENERGY) ASSESSMENT-

DETERMINE THE DEMANDS OF POWER CONSUMPTION:

 

The first step in designing a solar PV system is to find out the total power and energy consumption of all loads that needs to be powered by the solar PV system. For this assessment, we have to gather information regarding the electrical appliances (referred to as load) that will be powered by the PV system and estimate how much energy is required for the operation of load. The information to be recorded includes Name of Appliance, Type (AC/ DC). Quantity, Power rating and Number of hours the appliance will be used per day. From these data, we can calculate the total power of all connected loads and their daily energy consumption.

To determine the daily energy usage of an appliance, multiply the power rating of the appliance by the number of hours it will operate per day. It is normally expressed in watt hours (Wh) or kilowatt hours (kWh).

Appliances can either be DC or AC. An energy assessment should be undertaken for each type (AC and DC loads) by using the load (Energy) assessment form as shown below.

Load (Energy) Assessment Form

1

2

3

4

5

6

7

Sl. No.

Name of Appliance

Power (Watts)

No.

Total Watts (W)

No.of hours of usage/ day (h)

Energy consumed/ day (Wh)

1) 

 

 

 

 

 

 

2) 

 

 

 

 

 

 

3) 

 

 

 

 

 

 

Table.1

A system designer needs to gather load details from the customer and calculate the electrical energy usage. The problem is that, the customer may not want to spend the time determining their realistic power and energy needs which are required for successful completion of load assessment form. They just want to know only the cost of the PV system that is needed to power their lights and TV. A system designer can only design a system to meet the power and energy needs of the customer. The system designer must therefore use this process to understand the needs of the customer and at the same time educate the customer. Completing a load assessment form correctly does take time; you may need to spend enough time with the potential customer for completing the tables. It is during this process that you will discuss all the potential sources of energy that can meet their energy needs and you can educate the customer on energy efficiency.

  HOW TO FILL LOAD (ENERGY) ASSESSMENT FORM?

The load assessment form consists of 7 columns.

The first one is serial number

In the second column, you have to fill the name of appliance

e.g. CFL, TV, Fan etc.

Column 3 is Power in watts. It should be filled with wattage of the appliance. You can find the wattage of an appliance on the tag or plate located at the bottom or back of the appliances.  Or you can refer to the appliance owner’s manual. If an item is rated in amps, multiply amps by operating voltage to find the watts. Normally, the power rating will vary between 5 to 1000 Watt.

If there are more appliances of same type, then fill the Fourth column with the total number of that appliance. Thus, if a household is using 5 CFLs of 12W each, then this column is to be filled as 5.

In the fifth column, calculate the total wattage of each type of appliance by multiplying the wattage of individual appliance by number of appliance. i.e., multiplying column (3) by column (4). You have to do this for all type of appliances listed in the form.

Finally, add up all the values in column 5 to calculate the total power of all appliances connected in the household.

Sixth column should be filled with total number of hours for which that appliance will be used in a day. If there are more appliances of same type, but each operating for different hours per day, then take the average of number of hours of operation.

Seventh column is for estimating the energy required for each appliance per day. This is obtained by multiplying the total power rating (watts) by No. of hours of operation (h). i.e., multiplying column (5) by column (6).  The energy is expressed in Wh.

To find the total energy consumed by the household per day, add all the values of column 7.The monthly energy consumption can be calculated by multiplying the energy consumption per day by the number of days in that particular month.

An example for estimating the energy consumption of AC loads in a household is given in table.2 below.

Load (Energy) Assessment Form

1

2

3

4

5

6

7

Sl.No.

Name of Appliance

Power (Watts)

No.

Total Watts (W)

No.of hours of usage/ day (h)

Energy consumed/ day (Wh)

1)    

Fan

60

2

60 x 2=120

8

120 x 8=960

2)    

CFL

20

4

20 x 4 =80

8

80 x 8=640

3)    

Tube light

35

2

35 x 2 = 70

6

70 x 6 = 420

4)    

LCD TV

80

1

80 x 1 = 80

6

80 x 6 =480

 

Total

350W

 

2500Wh

Table.2

 

 STEP 2:

 

SIZING AND CHOICE OF ELECTRONIC COMPONENTS:

Once the load assessment process is complete, the next step is to choose suitable electronics for the PV system namely the Inverter and Charge controller.  The capacity of these components depends on the voltage and current of the loads.

A)  INVERTER:

Most of the appliances we use in our daily lives use AC current. But a solar panel produces DC current and batteries also generate DC current. Hence, we have to convert this generated DC power to AC for operating our AC loads. This conversion is made possible by using inverters. An inverter is used in the system where AC power output is needed. The capacity of the inverter is given in terms of Voltage x Current or VA. Sizing of inverter depends on the wattage of appliances connected to it.

For stand-alone PV systems, the inverter must be large enough to handle the total amount of power you will be using at one time. The inverter should be selected in such a way that it should supply the desired power to the load. The desired power for the load is the total power of connected load. Hence, the desired output power of the inverter should be equal to the total power of connected load. It is always better to choose an inverter having power capacity higher than the total connected load.

Now, we have to find out the power that must be supplied to the input of inverter for getting the desired output power. For a given input power, the output power from the inverter will depend on efficiency of the inverter.  Thus, the efficiency of inverter is considered while calculating the input power to the inverter. The input power, output power and efficiency of the inverter are related as shown below:

For example, consider the load assessment we have done in the earlier step. The total power of all AC loads is 350W.  Now, let us determine an appropriate inverter for this household having 350W AC load.

 (Take the inverter efficiency as 94%)

The total wattage of all AC loads

=  350W

Output power capacity of the inverter = Total wattage of all AC loads that needs to be powered

Therefore, the output power capacity of Inverter

=  350W

Inverter efficiency

=  94%

Therefore, the required input power to the Inverter

= (350 x 100) ÷ 94

= 372.34VA ≈ 400VA

 

ESTIMATION OF REQUIRED INPUT POWER TO THE INVERTER:

Load

Inverter

Total Power of load(W)

Output (W)

Efficiency (%)

Input (VA) =(Output x 100) ÷ Efficiency

350

350

94

(35 x 100) ÷  94 =372.34VA ≈ 400VA

Table.3

 

ESTIMATION OF REQUIRED INPUT ENERGY TO THE INVERTER:

Similar to the input power calculation shown above, we can calculate the input energy that must be fed to the inverter to get sufficient output energy to meet the demand of load.  If the total energy consumption of all connected loads is given, then the required input energy for the inverter is estimated as shown below:

An example of estimation of input energy at the inverter input is shown in table below:

Total Energy (Wh)

Efficiency (%)

Total I/p Energy (Wh)

2500

94

2500 x (100/94) = 2660

Table.4

SELECTION OF SUITABLE INVERTER:

There are many different makes and sizes of inverters in the market.  After estimating the power capacity of inverter, the next step is to see what is available in market. Inverters available in market will be designed for specific input DC voltages. Hence, the selection of inverter is important and all our system design (battery and PV module) should be done for this specific DC voltage.

In the above example, the estimated power rating of inverter is 400VA. From the standard ratings of inverters available in market, let us choose an inverter of 400VA with 12V DC input voltage and 230V AC output. Since the inverter can take only 12V DC at the input, the entire system voltage i.e., the batteries and PV module voltage should be fixed to 12V.

 

For the same inverter power rating, there can be different DC input voltages. It is good to choose an inverter with high DC voltage. This is because, for the same power flow, high voltage requires less current in the system. Also, lower the current flow, lower will be the power losses in the circuit. This require smaller sizing of wires and ultimately less cost to the system. However, one of the limitations in choosing inverters of higher system voltage is that the system requires more number of batteries.

 

B)  DC TO DC CONVERTER:

PURPOSE OF DC TO DC CONVERTER

In addition to the normal AC loads, if we are planning to power various DC loads by our solar PV system, then our system has to supply different levels of DC voltages as well for the proper functioning of each DC load. This can be achieved by using a DC to DC converter which will convert one DC voltage level to another DC voltage level. The DC to DC converter for a system is selected based on the input voltage levels of DC appliances that are planned to be connected to the system.

HOW TO SELECT A DC TO DC CONVERTER?

For the selection of DC to DC converter, first we have to list the input voltages of all DC appliances that are connecting to the system. In the case of multiple DC loads, the converter has to supply necessary DC voltages to all loads. Moreover, if the DC input voltage required by each appliance vary, then choose a DC to DC converter with multiple DC outputs. The input voltage to the converter will be the system voltage.

For e.g., if we are connecting 12V LED light, 12V DC Fan and 24V DC pump to the PV system, then choose a DC to DC converter having multiple outputs of 12V and 24V.

C)  SOLAR CHARGE CONTROLLER (SCC) OR MPPT:

In the previous chapters we have learned about charge controllers and its various types. A solar charge controller is typically rated against Amperage and Voltage capacities. A solar charge controller has to be selected as per the required input and output voltage and current of load as well as the battery. Make sure that the solar charge controller chosen has enough capacity to handle the current and voltages that are likely to be flowing in the system.

In the above example, the system specification is as follows:

Wattage (VA)

= 400VA

System Voltage

= 12V

Total energy (Wh)

= 2500Wh

Maximum system current

= 34A (400VA ÷ 12V)

 

In this case, the SCC or MPPT chosen should have output voltage and current handling capacity approximately to 12V and 34A respectively.

 STEP3:

DETERMINING THE SIZE OF BATTERY BANK:

 

Once the selection of electronic control circuitry is over, the next step is to determine the size of battery bank i.e., to find out the capacity, voltage, Ah ratings and number of batteries required for our PV system. The inverter and DC to DC converter will not have 100% efficiency and some amount of energy loss happens in these components. These losses have to be compensated by the battery bank. Sizing a stationary battery is important to ensure that the loads being supplied or the power system being supported are adequately catered for by the battery for the period of time (i.e. autonomy) for which it is designed. Improper battery sizing can lead to poor autonomy times, permanent damage to battery cells from over-discharged, low lead voltages etc. Therefore, we have to choose batteries in such a way that they should supply the power and energy required by the load for a definite period for which it is designed as well as be able to compensate the energy loss occurring in the inverter and DC to DC converter of the system.

BATTERY PARAMETERS & BATTERY SIZING:

In order to find out the size of the battery, we have to consider the following parameters of batteries.

·          System Voltage and Ampere-hour capacity of the battery

·          Depth of Discharge (DoD)

·          No. of days of autonomy

 

ESTIMATING AMPERE-HOUR CAPACITY OF THE BATTERY:

The most common measure of battery capacity is Ah, defined as the number of hours for which a battery can provide a current equal to the discharge rate at the nominal voltage of the battery. Ah capacity of the battery can be calculated by dividing the energy input to the inverter (Output energy from battery) by System voltage. 

 
   

 

Consider the above example where we need to supply 2660Wh energy to the input of the inverter and system voltage chosen is 12V.

Energy= Watt x hour = (V x I) x h

Therefore,

Battery capacity, Ah = (V x I x h) ÷ V   =Energy (Wh) ÷ System Voltage (V)

 

Here, the Ah capacity to be supplied to the inverter is estimated as follows:

Energy I/p to the Inverter (Wh)

System Voltage (V)

Ah capacity to be supplied

2660

12

2660 ÷ 12 =221.66Ah

Table.5

DEPTH OF DISCHARGE OF BATTERIES:

Depth of Discharge in short as DoD, is used to describe how deeply the battery is discharged. In many types of batteries, the full energy stored in the battery cannot be withdrawn (in other words, the battery cannot be fully discharged) without causing serious, and often irreparable damage to the battery. The Depth of Discharge (DOD) of a battery determines the fraction of power that can be withdrawn from the battery. For example, if the DOD of a battery is given by the manufacturer as 25%, then only 25% of the total battery capacity can be used by the load. Since DoD is not 100%, the actual Ah capacity of battery that is needed to supply required energy to the inverter should be higher than the estimated value.

In our example, the estimated value of Ah capacity is 221.66Ah and the actual Ah capacity of battery should be higher than 221.66Ah.

The actual Ah capacity of battery for a given DoD value can be calculated as follows:

Required Battery Capacity (Ah)

DoD (%)

Actual Ah capacity

221.66 Ah

60

221.66 ÷ 0.6 = 369.4Ah

Table.6

NUMBER OF DAYS OF AUTONOMY:

In standalone PV system, energy generated by PV modules is stored in batteries in order to supply the power to load during non-sunshine hours. The number of days of autonomy is the number of days that the battery must support the load even without any power generation from PV modules (i.e., during cloudy days or rainy seasons). Therefore, the more the number of days of autonomy, the bigger the capacity of the battery must be to store extra energy. If we want to store energy for one extra day, then our battery capacity should be double (one capacity for today and one for extra day) and if we want to store energy for two days then our battery capacity should be three times.

The Battery Ah capacity for 2 days autonomy can be estimated as follows:

Actual Ah capacity considering DoD (Ah)

No. of days of Autonomy

Required battery capacity (Ah)

369.4 Ah

2

369.4 x 2 =738.8Ah

Table.7

Thus, the total Ah capacity of battery can be estimated from energy input to the inverter by considering the DoD of the battery, System Voltage and No. of days of autonomy.

Total Ah capacity of battery= (Energy input to the inverter x (No.of days of autonomy +1)) ÷ (DoD x System voltage)

Thus using the above formula, we can calculate the total Ah capacity of the battery as,

Total Ah capacity of battery = (2660 x 2) ÷ (0.60 x 12) = 738.8 Ah

 

NUMBER OF BATTERIES REQUIRED:

The next step is to choose appropriate batteries available in the market and find out the number of batteries that are required to provide the required Ah capacity. The total number of batteries required can be found out by dividing the Total Ah capacity required for the system by the Ah rating of battery chosen for our purpose from the market.

Total No.of Batteries required= Total Ah capacity required ÷ AH capacity of chosen battery

For e.g. in our case, we need a battery bank of 738.8Ah capacity and with terminal voltage 12V. The next step is to choose suitable batteries available in the market.

Let us choose 12V, 150Ah battery for our purpose.

Then, the total number of batteries required=738.8Ah ÷ 150Ah

= 4.93 ≈ 5 batteries

Thus our system needs 5 batteries of 150Ah, 12V.

 

CONNECTION OF BATTERIES:

The input voltage to the inverter is 12V (System Voltage) and we have chosen 12V batteries. The number of batteries to be connected in series is calculated by dividing the System Voltage by Standard battery voltage. In this case, both the voltages are same and hence, we need to connect all the batteries in parallel. If our system voltage is 24V, then we need to connect two batteries (12V) in series to get 24V.

The number of batteries to be connected in parallel is found by dividing the total number of batteries by the number of batteries connected in series. In this case, it is 5 ÷ 1= 5.

 

ESTIMATION OF REQUIRED INPUT ENERGY TO THE BATTERY:

In practice, the battery efficiency may not be 100% and hence the entire energy supplied by the PV module may not be obtained from the output of battery bank. For a getting a particular output energy from battery (output to the inverter), the input energy to be supplied at the battery input is estimated by:

Input Energy = (Output Energy x 100)/ Battery Efficiency

In the above example, the energy to be supplied at the input of inverter is 2660Wh. Considering the battery efficiency as 90%, the energy required at the input of battery is,

Input energy of battery = 2660 ÷ 0.90= 2955.5Wh ≈ 2956Wh.

Total Energy (Wh)

Battery Efficiency (%)

Energy from SPV module  (Wh)

2660

90

2660 ÷ 0.90= 2955.5 Wh ≈ 2956Wh

Table.8

Fig.5.6 Diagram showing the Energy Levels at various stages of system

 

Step4:

Determining the size of Photovoltaic Module:

 

As shown in the energy flow diagram fig.5.2, the SPV module must supply enough energy to the battery, so that battery can supply enough energy to the inverter in order to supply the required energy to the load.

In practice, the battery efficiency may not be 100% and hence the entire energy supplied by the PV module may not be provided to the input of inverter. To compensate this loss, PV module should supply more energy to the battery input in order to provide the required energy to the inverter.

In the above example, the input energy of battery = 2955.5 Wh ≈ 2956Wh.

Input energy of battery= Output energy from SPV module

Thus, the energy from SPV module must be 2956Wh

 

NUMBER OF MODULES & POWER RATING:

The amount of solar radiation available at the site is a major factor that determines the number of PV modules required. The daily solar radiation is given in terms of KWh/m2/day. Typically solar radiation in India varies between 4 - 7 kWh/m2/day.

Consider the daily solar radiation at the location where our system is installed as 5.5 kWh/m2/day. As we have learned in earlier chapters, at STC (Standard Test Condition), the solar radiation intensity of solar PV modules is 1000 W/m2 or 1kW/m2. Hence, the daily solar radiation of 5.5 kWh/m2/day can be represented as:

Daily solar radiation= 5.5 kWh/m2/day = 5.5kW x h/(m2 x day)

= 5.5 h/day x 1kW/m2

In the above equation, 1kW/m2 is the solar radiation intensity at STC

Therefore,Daily solar radiation = Equivalent sunshine hours/day x Solar radiation intensity at STC

Thus, we can say for 5.5 kWh/m2/day, the solar radiation is 1kW/m2 for 5.5 hours.

PV MODULE SIZING:

In our example, the total energy required is 2956Wh/ day = 2956 W x (h/day)

The daily solar radiation is 5.5h/day.

The required power of PV modules can be estimated by dividing the total energy required per day by the value of daily solar radiation.

Required Power of PV modules = Total Energy Required per day (Wh/day) ÷ Daily Solar Radiation at the site (h/day)

Therefore, the required power of PV modules =

= 2956Wh/day ÷ 5.5h/day = 537.45W

Let us consider the PV module power loss as 25%. 

Then the solar PV module required=537.45 W x 1.25=671.81W

Energy from SPV module  (Wh)

Daily sunshine hours

PV module loss (%)

Wattage of SPV module  (W)

2956

5.5

25

(2956 ÷ 5.5) x 1.25

= 671.81 ≈ 672W

Table.9

Thus, we need SPV modules of 672 watt peak in our system.

NUMBER OF PANELS REQUIRED:

The next step is to choose appropriate PV modules available in the market, find the number of modules required and how it is to be connected.

In order to find the number of PV modules, divide the total power required from SPV module by the total power of single PV module.

Total No.of PV modules= Total estimated power of PV module ÷ Power of single module

 
   

Let us choose a 100Wp, 12V PV module for our purpose.

Then, the Total number of PV modules required = 672 ÷ 100 = 6.72 ≈ 7 PV modules.

 

NUMBER OF PANELS TO BE CONNECTED IN SERIES (STRING):

If system voltage is higher than voltage of PV module, then we need series connection of PV modules in order to increase the voltage to the level of system voltage. The number of SPV modules to be connected in series (which is known as string) is calculated by dividing the system voltage by voltage of SPV module.

No.of PV modules to be connected in series = System Voltage  ÷ Voltage of SPV module

In the above example, system voltage is 12V and standard SPV module voltage is 12V.

Number of PV modules to be connected in series (strings) =

= System Voltage ÷ Voltage of chosen SPV module  

=12V ÷ 12V = 1

Thus, in our system, the system voltage and SPV module voltage are matching.

NUMBER OF PANELS TO BE CONNECTED IN PARALLEL (ARRAY):

Number of PV modules to be connected in parallel (array) =

Total number of PV modules required ÷ Number of PV modules in series

In the above case, the total number of PV modules required is 7 and Number of modules to be connected in series is 1.

Then the number of PV modules to be connected in parallel =  7 ÷ 1 = 7

 

 

 

 

 

 

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