Lesson 6: System Design Processes for Grid-connected and Stand-alone Systems

Overview

Lesson 6 Scenario

You work as a solar system designer for a solar company. You have two potential clients who want to install PV systems. The first client is interested in a grid-connected system while the other wants an off-grid system. After reviewing data collected by the sales representative from the site, your task is to accurately size these systems.

What do you look for when deciding the system size in regards to the load demand? Is there a difference between grid-connected and off-grid PV system design? What are the steps to designing each system?

In this lesson, we will find answers to these questions and we will discuss topics ranging from load analysis to PV system design. This lesson prepares solar professionals to become PV system designers. Sizing PV systems is similar to art, where using the right tools ensures the PV system is sized to function properly.

Learning Outcomes

At the successful completion of this lesson, students should be able to:

Compare sizing strategies and methodologies for different PV system types.
List factors that affect sizing a PV system.
Calculate load requirements for different PV system types.
Apply tools for sizing grid-connected and stand-alone PV systems.

What is due for Lesson 6?

Lesson 6 will take us one week to complete. Please refer to the Calendar in Canvas for specific time frames and due dates. Specific directions for the assignments below can be found within this lesson and/or in Canvas.

Complete the following Lesson Assignments:

Read through the Lesson Content
Complete the Required Reading Assignments:
- Chapter 9, Photovoltaic Systems by James P. Dunlop (text)
Look over the Recommended Reading:
- Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors [1]
Complete the Lesson 6 Activity
Participate in the Lesson 6 Discussion
Take the Lesson 6 Quiz in Canvas

Questions?

If you have lesson specific questions, please feel free to post to the Lesson 6 Questions discussion forum in Canvas. While you are there, feel free to post your own responses if you, too, are able to help a classmate with a question. If you have questions about the overall course or wish to share and discuss any "extra" course related commentary (interesting articles, etc.), please feel free to post to the General Questions and Discussion forum.

System Sizing Overview

In previous lessons, we learned about PV modules, batteries, and inverters in terms of performance characteristics and main parameters. At this point in the class, we will introduce the concept of sizing PV systems. System sizing involves the detailed calculations of the energy produced by the PV system and matches it with the desired energy demand. We will put the knowledge we learned together in addition to introducing some sizing tools that can be used to determine the exact number of modules needed to form a PV array. The main parameter that PV designers name as a goal when designing a PV array is the load energy demand or energy usage in (kWh).

When describing a PV system in terms of components, it is logical to use the energy flow path from the array side to the load side. However, when sizing a PV system, it is necessary to consider the energy demand before considering the PV supply side. For that reason, PV system sizing starts at the load side and proceeds backward to the PV array side. For example, PV designers need to know the energy usage (kWh) before choosing the PV array size.

Limitations to sizing

Not all clients/sites are ready to accommodate PV arrays. That can be due to one or more of the following factors:

Factor 1: Space availability for the PV array (roof square footage/meter)
Factor 2: Owner’s budget
Factor 3: Annual energy consumption (client's energy bill in kWh)

PV systems are generally sized to maximize the solar utility for a client within these aforementioned limits. For the purpose of this class, we will focus only on the last factors that influence the sizing of any PV system. Before diving into the energy demand, lets elaborate more on these factors.

Factor 1: Space availability for the PV array (roof square footage/meter)

Before installing a PV system, it is important to make sure that the site is suitable for installation by considering the following factors:

Shade-free (Sites are best optimized when there is no shading on the PV array.)
South-facing (In the northern hemisphere, the sun path is generally leaning towards the south. The opposite applies to the southern hemisphere where the sun appears to be towards the north of an observer.)
No obstruction (Sites need to have minimum obstructions before installing PV arrays. Obstructions may include chimneys, gables, HVAC units on rooftops, and so on.)

Factor 2: Owner’s budget

Budget is an essential factor that plays a huge role in the design process. It varies with available rebates and incentives at each location. As we mentioned previously, this class will not consider the finances of PV systems, as it is covered in other RESS classes.

Factor 3: Annual energy consumption (client's energy bill)

When sizing a PV system, It is important to consider the following criteria:

PV system size to meet 100% of client's annual energy usage kWh
PV system size to be permissible by utility interconnection rules

In the next topic, we will discuss how annual energy demand is estimated and how to go about understanding the client’s energy bill.

Energy Demand Analysis

As the name implies, the energy can be estimated at different levels, and that depends on the application intended.

For example, Grid-connected PV systems are more flexible to solar energy intermittence since the utility grid is used as a backup source when more energy demand arises. With Stand-alone PV systems, since energy is delivered instantaneously to the load without utility grid backup, the load requirements are less flexible to energy supply. That is a consideration when sizing PV systems.

For all PV systems, the main sizing factor is energy consumption. That can be estimated or calculated depending on the energy data availability. The following scenarios summarize types of data provided to the PV designers before sizing the PV system:

If the client can provide 12-month energy bills, the designer can then evaluate these bills to come up with the average monthly energy consumption and total annual energy consumption.
If the client doesn’t have all 12 month bills but he/she can provide energy bills of some months, the designer should come up with a creative way to estimate the total annual energy based on the provided data.
If the client doesn’t have any energy bill records at all, the designer should then run the estimation based on the client’s energy loads to come up with the total annual energy consumption. That is referred to as load analysis and it will be discussed in this topic.

Energy Estimation

As seen earlier, energy estimation can be as easy as reading an energy bill, or it can be as challenging as running a load analysis. How do we go about it in either case?

Analyzing Energy Bill:

The total energy consumed can be calculated by using all monthly energy bills (in kWh) for the entire year. This is considered the most accurate method to estimate the average monthly and total annual energy consumption based on real data provided by the client. However, this method doesn't provide consumption detail such as daily energy demand or hourly energy demand since the energy readings on the monthly bill are usually taken once a month.

Load Analysis:

In order for a designer to learn more about the daily or hourly energy demand of a property, more detailed calculations are required to achieve that task and that is usually done at the load level. The hourly and daily consumption can be measured for any property; however, this method requires using energy analyzing devices at the meter side for a significant period of time. This method will generate the most accurate energy consumption data. However, it requires more time and budget.

Since power demand is usually stated on the rating of each device/appliance individually, there should be an easier way to estimate daily energy demand. We learned in "Electricity Basics" in the Orientation that power is not the main sizing parameter since the same load can consume zero energy if it is turned-off or it can consume a lot of energy when it is turned-on for a long period. Energy consumption is based on the power demand over a period of time.

Since most loads don’t run continuously for the entire day, operating time is another parameter that should be considered when estimating the total energy consumption of a device.

When running load analysis calculations, it should be noticed that there are two types of loads:

AC loads that requires AC power to run. (Note: this is what most of our loads are.)
DC loads that can be run directly from the PV system.

Power generated from the PV array is DC power, and in case there are AC loads, power conditioning units are used as described in previous lessons. Since PCU is not 100% efficient, it will consume some energy. That should be added to the load analysis as an additional energy usage. The energy consumption of an inverter is estimated by its conversion efficiency.

Example:

As a result, the total required energy that should be provided by the PV array is calculated as follows:

E_{s d c} = \frac{E_{a c}}{E_{f f}} + E_{d c}

Where:

E_sdc is the required daily system DC (Wh/day)

E_ac is the AC energy consumed by load (Wh/day)

E_ff is the inverter efficiency

E_dc is the DC energy consumed by loads (Wh/day)

Note:

To learn more about load analysis, please refer to the required reading of Chapter 9 in the text.

Grid-connected system sizing

Grid connected systems, or utility interactive system design, is very straight forward. We saw an example of PV sizing when we did the basic simulation exercise in Lesson 1, when we learned about PVWatt and SAM software. As can be noticed, PV annual energy production varies according to the location of the system that is provided by the solar radiation resources of each specific location. These systems are usually designed to either meet 100% of the annual energy demand of the load or only to offset a percentage of the energy usage that the client desires.

Example:

A 1kW PV system located in State College, PA, can generate 1,231 [kWh/yr] at 30° tilt and true south orientation (180° Azimuth).

Assuming the client's energy consumption is 6500 kWh/yr, What is the PV system size?

P V_{s i z e} = \frac{6500}{1231} = 5.28 [k W]

Another way to calculate PV system size is to use the average daily solar radiation, also known as Peak Sun Hour (PSH), of that location. PSH can be defined as the equivalent number of hours per day when solar irradiance averages 1,000 W/m². For example, five PSH means that the energy received during total daylight hours during a day equals the energy that an irradiance of 1,000 W/m2 would have been received for five hours.To estimate the PV system size, we divide the energy consumption by the number of days per year (365 days/yr) to find the daily average energy consumption, and then divide that by the PSH of the location. The calculation can be done as follows:

P V_{s i z e} = \frac{E_{u s a g e}}{365 \times P S H}

Where:

E_usage is the annual energy consumption in [kWh/yr]

PSH is the Peak Sun Hours in [h / day], which is equivalent to the solar insolation in [kWh / m² / day].

Example:

In State college, PA, we have PSH of 4.22 [h / day].

P V_{s i z e} = \frac{6500}{365 \times 4.22} = 4.22 [k W l m^{2}]

Since the PV system includes losses (such as inverter losses, cables losses, mismatch, soiling, degradation andso on), these factors can reach up to 25% of system losses. In other words, the actual size of PV system is: $P V_{s i z e} = 4.22 \times 1.25 = 5.28 [k W]$

Note:

To find the Peak Sun Hour for a location in the US, you can use PVWatt data or visit the Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors website. [1]

Sizing Considerations

Consideration 1: Module Selection

Modules can form strings, as we learned in Lesson 2. In this case and after we size the system, it is important to find the required number of modules as follows:

Array Watts / Module = Number of modules

Some array size may be slightly different from the calculated system size due to the availability of modules sizes:

Example:

$5200 / 200 W(per module) = 26 modules$ . We can split them in TWO strings of 13 each
$5200 / 240 W(per module) = 21.7$ . We need to round it up to 22 modules. We can split them in TWO strings of 11 each

Designers should consider the derate factors of the module when sizing a PV array, such as: modules’ power tolerance, power degradation with time, temperature coefficients that lower the power of the module, and array wiring mismatch.

Consideration 2: Inverter Selection

As discussed in Lesson 4, inverters vary by voltage ranges and efficiency. Designers should consider inverter efficiency and MPPT efficiency when sizing PV systems.

Consideration 3: Site

Designers should account for any environmental factors that may contribute to losses in the PV array when sizing the system, such as soil, snow factors, or shading losses.

Note:

To learn more about interactive system sizing, please refer to the required reading of Chapter 9 in the text.

Stand-alone system sizing

Critical design analysis

A PV system cannot generate constant energy for the entire year, but a stand alone PV system should be able to supply loads during any month of the year, and since solar energy generation varies by month, a PV designer should take into account the critical design value for the PV system. For example, if the application requires more energy during the winter, where low insolation occurs, then the PV system should be sized to meet the load requirement of that specific month or season.

Critical design month:

When the PV system needs to meet different load requirements throughout the year, the month with the highest design ratio is referred to as the critical design month. It is taken into account the worst-case scenarios associated with the lowest insolation and highest load demand. We can analyze this design ratio at three tilt angles: Lattitude, Lattitude +15, and at Lattitude -15 degrees. As we said the highest ratio value will be the critical ratio and the month associated with it is the critical desighn month.

Considerations:

Since array orientation has a significant impact on generated energy, the orientation should be chosen to match the critical design month. For example, if a 15° tile produces more energy during the critical design month when compared to a 25° tile, a designer should consider the 15° to optimize the system design as long as it doesn’t affect other design months' values. The cost associated with the lower tilt is another factor to take into account when selecting the racking materials.

System design voltage

PV system DC voltage link is determined by the battery bank in stand-alone systems. As we discussed earlier, battery voltage can be 12V, 24V, or 48V. The voltage level changes depending on system size. As a rule of thumb, small PV systems are usually 12V systems, and larger systems are preferred to be 48V to handle more current. Some very large systems can be 120V, but that is considered a special case.

System Availability

Since the solar irradiance is not always available, stand-alone systems need to be sized to meet load demand for the entire year, and that is expressed by system availability, which is the percentage of time that a stand-alone system can meet the load demand within the period of a year. It is determined by isolation and autonomy. Autonomy is the amount of time the load will be supplied from the battery bank by itself and is expressed in days. For example, 95% availability (3 days of autonomy assumung PSH is around 5.0 for that location) means that the system cannot meet the load demand for 5% of the time.

Note:

This figure illustrates that local sun hours for any location along with the desired sytem availability determine the system autonamy (in days). It is also important to know that the system availability depends on how critical the load application is. For critical loads, 99% is considered acceptable (10 days of autonomy if your average PSH is around 4.0)

You can refer to chapter 9 in textbook to read more.

Battery Sizing

After we learned in Lesson 3 about the main parameters of batteries, we established that batteries are used to store energy for later use. A stand-alone system is a perfect application.

Battery bank required output capacity

Considering the daily energy demand during critical design month and desired days of autonomy, the batteries should be able to provide energy to the load for all of the autonomy days.

Example:

The required battery capacity is calculated as follows:

B_{o u t} = \frac{E_{c r i t} \times T a}{V_{s d c}}

Where
B_out is the required battery bank output capacity (Ah)
E_crit is the required daily system energy demand for the critical month (Wh/day)
T_a is the days of autonomy
V_sdc is the nominal DC-system voltage

Battery bank rated capacity

As we established in Lesson 3, no battery can be completely discharged, and that is referred to as allowable DOD that a battery cannot exceed. It ranges from 20%-80% depending on battery type. Also, the operating temperature affects the available capacity the battery can deliver. Low temperature with high DOD can reduce the available battery capacity. Finally, the discharge rate is a main factor that determines the available battery capacity at certain temperature. This is expressed as a derating factor to the available capacity.

Example:

To put all factors together, we can write:

B_{r a t e d} = \frac{B_{o u t}}{D O D_{a} \times C_{t, r d}}

Where:

B_rated is the battery bank rated capacity (Ah)

B_out is the required battery bank output capacity (Ah)

DOD_a is the allowable depth of discharge

C_t,rd is the temperature and discharge -rate derating factor

Array sizing

A PV array should be sized to supply enough energy the meet the load demand at the critical design month while accounting for the system losses. This will ensure that system availability is high and the battery bank is charged.

Rated array output

Similar to grid-connected systems, array size can be determined using the peak sun hours of the location. However, since we have different DC system voltage depending on the system, it is more desirable to calculate the array current. Furthermore, off-grid systems include batteries. These batteries are not 100% efficient, so this should be taken into account.

Example:

I_{a r r a y} = \frac{E_{c r i t}}{η_{b a t t} \times V_{s d c} \times T_{P S H}}

Where:

I_array is the required maximum power current [A]

E_crit is the required daily system energy demand for the critical month (Wh/day)

η_batt is the battery efficiency

V_sdc is the nominal DC-system voltage

T_PSH is peak sun hours for the critical design month (hr/day)

There are factors that reduce the output of any PV array.

Note:

This section is thoroughly covered in Chapter 9 in the required reading textbook.

Lesson 6 Activity

Activity

Details

Assignment

Assume you have a client in State College, PA who is interested in a PV system.

Scenario 1:

The client is connected to the Utility Grid and he is interested in saving money on his energy bill.

Scenario 2:

The client currently runs his property based on a Diesel Generator and he is looking for a cleaner and alternative option. The Diesel Generator system availability is currently 95%.

You are given the following information in regards to monthly electricity usage:

Electricity Monthly Usage
Month	Usage in kWh
January	700 kWh/month
February	750 kWh/month
March	769 kWh/month
April	800 kWh/month
May	960 kWh/month
June	1000 kWh/month
July	1100 kWh/month
August	1050 kWh/month
September	860 kWh/month
October	850 kWh/month
November	830 kWh/month
December	720 kWh/month

Deliverable

For each of the scenarios, use the appropriate tools or calculation methods to find the following:

The actual PV system size required
The type of PV system
The corresponding Tile and Orientation of the PV array
Critical design month, when applicable
The DC system voltage, when applicable
The required battery capacity, when applicable

Use the following values when applicable:

DOD 50%
Battery Efficiency 85%

Write up a report with your findings. The report should be saved as a PDF and it should be no more than two double-spaced pages in a 12 point font.

Submission Instructions and Grading

Please visit the Lesson Activity [2] page for submission instructions and grading information.

Lesson 6 Discussion

Lesson 6 Discussion
Activity	Details
Assignment	Post original entry: We talked in class about sizing processes for both grid-connected and stand-alone PV systems. However, based on the Lesson 1 Discussion [3], we learned that PV systems classification can include more a specific market sector such as: Solar installation for rural off-grid application Residential rooftop for grid-tied application Utility Scale PV installations Shared solar for communities In this lesson, we ask our solar professionals to discuss how sizing processes might vary for different market sectors. Discuss why there might be specific sizing considerations for one of the options. Support your discussion with facts (you may research the same solar installation example you selected for the Lesson 1 Discussion). Post comments: Respond to two different opinions of others' posts. (For example, if you choose Option 1, you need to respond to one post for Option 2 and another post for Option 3 or 4.)
Requirements, Submission Instructions, and Grading	For more detailed instructions about the discussion component of this course, including how you will be graded, please visit the Discussion Activity [4] page.

Summary and Final Tasks

In this lesson, we covered a variety of topics starting from different methods to estimate energy demand including use of energy bills and detailed load analysis. By now you can assist your client in the scenario by providing an accurate PV system sizing taking into account factors that affect the energy output for both grid-connected and stand-alone systems. Furthermore, as a designer, you have the right tools to optimize your stand-alone system design by choosing the right critical design month and parameters.

In the next lesson, we will focus on the standards and regulations that govern PV system design and installation such as building, fire, and electrical building codes.

Reminder - Complete all of the Lesson Requirements!

You have reached the end of this lesson. Before you move to the next lesson, double-check the list on the first page of the lesson to make sure you have completed all of the requirements listed there.

Lesson 6: System Design Processes for Grid-connected and Stand-alone Systems

Overview

Lesson 6 Scenario

Learning Outcomes

What is due for Lesson 6?

Complete the following Lesson Assignments:

Questions?

System Sizing Overview

Limitations to sizing

Factor 1: Space availability for the PV array (roof square footage/meter)

Factor 2: Owner’s budget

Factor 3: Annual energy consumption (client's energy bill)

Energy Demand Analysis

Energy Estimation

Analyzing Energy Bill:

Load Analysis:

Example:

Note:

Grid-connected system sizing

Example:

Example:

Note:

Sizing Considerations

Consideration 1: Module Selection

Example:

Consideration 2: Inverter Selection

Consideration 3: Site

Note:

Stand-alone system sizing

Critical design analysis

Critical design month:

Considerations:

System design voltage

System Availability

Note:

Battery Sizing

Battery bank required output capacity

Example:

Battery bank rated capacity

Example:

Array sizing

Rated array output

Example:

Note:

Lesson 6 Activity

Scenario 1:

Scenario 2:

Deliverable

Lesson 6 Discussion

Post original entry:

Post comments:

Summary and Final Tasks

Reminder - Complete all of the Lesson Requirements!