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STAND ALONE SOLAR PV SYSTEM

What is a stand alone solar system?

This is a type of system which is not connected to electric grid. We have a PV array to

convert sunlight into electricity and then we store this energy in a battery as chemical energy.

This stored energy is then utilized when needed. This system is also known as off-grid system.

What are the components of stand alone system?

We will take a look at the processes involved. We have to convert sunlight into electricity

so, we need a PV array. This electricity has to be stored, so we use a battery. We cannot directly

feed electricity from PV module to battery. There is a component which keeps both PV module

and battery happy, that is a charge controller. This ensures that our battery is not overcharged

and at the same time operates our PV module at its maximum efficiency (not all charge control-

lers).

Are these the only components used? The answer is NO. We have a big task of powering

all our appliances. We hardly have any DC powered appliance at home. All our appliances like

TV, Computers, and Microwave etc. are AC powered. So, we have to convert DC electricity

from battery into AC by using an inverter.

Applications of stand alone systems:

Some commonly found systems are calculators, wristwatches, cell phone towers, road

signals, street lights, road signs, water pumping system, ATM machines, portable systems to

name a few.

Our objective is to learn how to design this system using PVSyst and I will stick to that

while providing occasional references and examples. If you don’t have PVSyst installed in your

computer, please use this link http://www.pvsyst.com/en/download and download a demo ver-

sion.

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Introduction:

Open PVSyst and in the main screen, you will find three options,

1. Preliminary design,

2. Project design, and

3. Tools

1. Preliminary design:

This helps you design a system with fewer options like type of system, location, tilt angle

etc. In our case of stand-alone system you can choose only location, tilt angle and load. It will

compute and tell you how big a system you will need. This is just like choosing the color of your

car interiors. You have very few options (forgive me for poor example).

2. Project design:

Using this option you can modify various properties of your system. You can choose the

type of module, make, peak watt, battery type, module orientation, shading etc. this is like an op-

tion to choose the color of your car interiors along with fabric type, type of cushion, seat belt etc.

3. Tools:

This section is used to feed user data’s like adding a new site, module, inverter etc. We

will discuss this section in detail at later stages as this is very vital.

Fig1. Main Screen

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Design:

We will start with project design option. When we learn to use all features then it is easier

to use the simple options. Preliminary design will only give you an approximate overall picture

of your project and does not help much in learning phase.

Step1. Choose Project design under Option and then click on Stand alone under System. Click

OK and you will be taken to next screen.

Step2. In this screen you will see options like,

1. Project,

2. Orientation,

3. Horizon,

4. Near Shadings,

5. System,

6. Module layout,

7. Simulation, and

8. Exit

Fig2. Design screen

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1. Project:

Click on the project button you see in the design screen. This screen lets you feed basic

details about our project. It includes details of location, solar irradiance, customer name, project

name etc.

Fig.3 Project details screen

Step1. Use the New project button to create a new project.

Step2. You will be prompted to enter project’s name, customer name, and other miscellaneous

details. When you are done with this step, please press Site and Meteo

Step3. Now choose the country for which you want to design PV system from drop down list

box.

Step4. Now choose site from the other drop down list box. Once you have selected your site,

please check the meteo file’s name.

Step5. If your site is not listed, you should add them from tools option as I mentioned earlier. I

will demonstrate how to add new sites at the end of this document.

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Site selection is to feed PVSyst with geographical coordinates of the location. For

detailed analysis we need hourly irradiance values. This hourly value is stored in

Meteo file.

Step6. Once you finish feeding values in this page, please click next. Now you will reach albedo

value page. Albedo value is the reflecting power of the surface. You can see a table on

the right side of the same page. Depending on the surface where your PV modules are lo-

cated, albedo value change. If you are not sure, better take the default value of 0.20.

Fig4. Albedo screen

Step7. Once you finish albedo value, move onto Site-dependent design parameters. You will see

4 different temperatures. What are these? Why should we consider these? How will these

affect our design?

“We all know that temperature affects power output of PV modules. Temperature

coefficient determines this loss percentage and varies for different modules. The

above indicated temperatures are the location ’s temperature.

Lower temperature for VmaxAbs limit:

This is the lowest temperature that you will have in your site. This value is

usually a historic value. When module temperature is low, it performs better. We

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will get higher voltage from the module at lower temperatures. We should make

sure that inverter’s or charge controller’s maximum input voltage is greater than

this value.

E.g.) We have 3 modules connected in series. They have a voltage output of

40volts each at -10˚ C. So when you connect these modules in series you will pr o-

duce a maximum of 120 volts. Maximum input voltage range of your inverter or

charge controller should be more than 120 volts. This is one aspect of system si z-

ing.

Winter operating temperature:

This is same as the above but it is not historic lowest temperature of the

region. Instead it is average temperature during winter .

Summer operating temperature:

This temperature is the opposite of winter operating temperature. It is the

maximum temperature of the location in summer. Rise in temperature results in

significance decrease in output voltage. This reduced voltage from the PV array

should not go below the minimum input voltage of inverter or charge controller.

E.g.) We have 3 modules connected in series. They have a voltage output of 25

volts each at 60˚ C. So when you connect these modules in series you will produce

a maximum of 75 volts. Minimum input voltage range of your inverter or charge

controller should be less than 75 volts. This is one aspect of system sizing.

Usual operating temperature:

This temperature is the average ambient temperature of the location. You

should also understand that temperature of PV module is not same as ambient

temperature. These are just like us; our body temperature is not same as ambient

temperature. We can calculate the module temperature from ambient temper a-

ture and measure the loss due to temperature. I will cover this in detail in Array

loss section.

Step8. After you enter all the values, click OK and save your project.

Stpe9. Click OK or next in all the screens that appear until you reach the design screen like Fig2.

Step10. Now choose orientation option. These features will help you position your modules the

way you want and also check their performance.

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2. Orientation:

In this section you can determine the tilt angle, type of orientation, and check the perfor-

mance instantly.

Fig5. Orientation tools window

Before adjusting the parameters in this window, it is necessary to know what they mean.

Different field types are,

1. Fixed tilted plane,

2. Seasonal tilt adjustment,

3. Tracking 2 axis,

4. Tracking 2 axis, NS frame,

5. Tracking 2 axis, EW frame,

6. Tracking tilted or horizontal, NS frame,

7. Tracking, horizontal EW axis,

8. Tracking, vertical axis,

9. Tracking, sun shields,

10. Double orientation,

11. Unlimited sheds, and

12. Unlimited sun shields

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What is tilt angle?

It is the angle which our PV module makes with the surface. I have explained the necessi-

ty for tilted module in my video.

What is azimuth angle?

Azimuth angle indicates us, the location of sun in east west axis. It is the angle which sun

makes with the line connecting South and North Pole.

In the case of PV module, if the module surface is exactly facing South or North Pole,

then azimuth is said to be zero. Else, it is whatever angle the surface makes with South or North

Pole.

1. Fixed tilted plane:

This type of orientation is very common. This does not have any tracking mechanism.

You fix both tilt and azimuth angle during installation. You can use the optimization option to

make the system suitable for summer, winter or year round performance. If you want to use this

option, use steep angle for winter as sun remains lower in the sky and use lower angles for sum-

mer as sun reaches high in summer. If you want year round performance, use latitude of the loca-

tion as your tilt angle (equatorial regions).

As you change the tilt angle and azimuth angle you can see the performance in meteo

yield section in the window. Use this to determine the optimum tile and azimuth angle.

2. Seasonal tilt adjustment:

This type lets you choose two tilt angles. Lower angles for summer and steep angles for

winter. Everything else is same as previous type. You can choose your winter months using the

check boxes provided.

3. Two axis tracking:

Dual axis tracking follows both altitude and azimuth angle of sun and adjusts’ PV mod-

ules tilt and azimuth angle respectively. By doing this we can get rid of incidence angle losses. I

will cover this loss in detail in later sections. You can set the angle for lower and upper limit for

rotation of PV module.

I am looking for more solid explanations for backtracking and concentration arrays. I will

cover this in next material as I don’t want to overload with lot of details.

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4. Tracking two axis, NS frame:

Tracking method is similar to dual axis tracking. Here you have more than one PV mod-

ule and you can set the tilt angle of the entire frame. Imagine your modules are placed in a slope.

You can feed that slope angle of the surface as frame tilt angle. Also, the modules are arranged in

North-South axis which means PV module will be facing either North Pole (site in southern

hemisphere) or South Pole (site in northern hemisphere).

5. Tracking two axis, EW frame:

This orientation is similar to the above but PV modules are placed in East-West axis.

6. Tracking tilted or horizontal, NS frame:

In this tracking system, Tilt angle is kept constant and only azimuth angle of the PV

module arrangement varies according to sun’s azimuth angle. You can setup azimuth rotation

limits using the options provided in the window.

7. Tracking, horizontal EW axis:

In this tracking system, azimuth angle is kept constant and only tilt angle of the PV mod-

ule arrangement varies according to sun’s altitude angle. You can setup tilt angle rotation limits

using the options provided in the window.

8. Tracking, vertical axis:

In this tracking system, Tilt angle is kept constant and only azimuth angle of the PV

module arrangement varies according to sun’s azimuth angle. You can setup azimuth rotation

limits using the options provided in the window.

9. Tracking, sun shields:

In this we have only one angle to adjust. That is the angle made by façade with wall or

side of the building. You can also set the tilting angle limits.

10. Double orientation:

In this a single solar farm is divided into two separate sections. The only differences be-

tween the sections are their tilt and azimuth angle. These are fixed orientations and no tracking is

used.

11. Unlimited sheds:

This is fixed tilt and azimuth angle arrangement but widely used in designing solar farms.

You can define the number of rows of PV modules, using number of sheds option. In shed pa-

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rameters, you can feed the distance between two rows using pitch option. Collector band width

defines the height of PV module. Remember, larger the module height, larger the shadow length.

Inactive band defines the portion of the structure which is not covered by PV module.

Fig6. Unlimited sheds orientation

Loss due to shadow effects is not dependent on the percentage of PV module that is shad-

ed. If you want more accurate shadow analysis, use the electrical effect. Electrical effect in shad-

ing calculates loss based on the arrangement of cells in the module. If one cell is shaded in a

string, this considers the entire string to be shaded. You can define the size of each cell and also

the number of strings in a module. Click on the show optimization button and check the shadow

loss percentage. This value is instantaneous and the annual shadow loss percentage will be more

or less the same.

12. Unlimited sun shields

This is similar to unlimited sheds but only the orientation is different. Use all the parame-

ters as explained above.

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Fig7. Electrical effect shading

Fig8. Shadow optimization graph

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I will cover Horizon and Near shading section in a separate file. It is good to have them

as a separate section.

5. System:

In this section we will define our loads. I will explain each and every section of this win-

dow.

Fig9. Daily use of energy

Consumption definition:

You can feed loads as common for the entire year, depending on the season as we con-

sume more energy during winter and also based on months. For instance, we consume more en-

ergy during summer breaks as we stay home long, and tend to use some appliance. You can also

define weekend system or just regular system.

Model:

If you have a predefined energy consumption (load) file, which you created and saved,

you can import them using load option and selecting the appropriate file.

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Daily consumptions:

Analyze your daily usage and fill this section. As we know, we will not operate big appli-

ances like, electric stove, electric heater, and air conditioners from stand alone PV systems. Try

not to include them as these will lead to enormous array size.

Hourly distribution:

We can use the hourly distribution to set the power consumption for each and every hour.

If you check the evening usage option, it will set TV and lighting usage to maximum in the even-

ing than during the day. This helps in determining the wear and tear of the battery.

After you feed all details, please press next to select the type of module, battery etc.

Fig10. System design

In the top section of this page you can enter battery voltage, LOLP, and number of backup days.

Battery voltage:

This determines the voltage of battery bank. Depending on the load and applications we

choose this battery voltage.

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LOLP:

This is Loss of Load Probability. This defines what percentage of load cannot be supplied

by this PV system. This depends on the application of your system. If you are designing system

for a rural hospital, then there is zero tolerance, so LOLP is set to 1%.

Backup days:

This defines the number of days, you need backup. Suppose your daily consumption is

200Wh, and you need backup for 3 days, then your system should produce 600 Wh in a single

day. This increases the size of PV array and also battery bank.

Battery set:

This is used to select the battery that you want to use. You know how much Amp hours

we need. So based on that value, number of batteries increase. You should arrange battery bank

using basic series, parallel connection. Remember, in series connection voltage adds up and in

parallel connection, current adds up. So, even if we don’t have enough amp hour battery, we can

connect them in parallel and make it possible.

Module selection:

Select the module from the drop down list box. If you don’t find the module which you

want, you can add them to database by using tools option. Depending on the peak watt and volt-

age of the module, PVSyst will calculate the number of modules required and the series, parallel

connections.

Regulator:

Now click next to choose the charge controller (regulator). If you know the type of

charge controller you are going to use, select them from drop down list, else simply choose de-

fault regulator.

If you choose any type of regulator, operating mode is updated from inbuilt data. If you

choose default regulator, you have to choose from either of three options. More information will

be added for this.

Please leave generator section, empty. We are not going to include any backup generator, for

now.

Array losses:

This will be explained in next document. This should be studied in detail.

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Please press ok. Now press the simulation button, and press simulation again. Once the simula-

tion is complete you can view the report or even print and analyze the system performance.

Fig11. Simulation page

Videos:

1. Adding new site to PVSyst from TMY3 data.

2. Explanation of various sun angles

3. Reasons for inclined module orientation.

Exercise1.

Design a system for your room and mail me the doubts you have. I will try my best to clarify be-

fore we move to next topic. M mail id: Raam_Perumal@student.uml.edu