AE 868
Commercial Solar Electric Systems

Grid-connected system sizing

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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 size = 6500 1231 =5.28[kW]

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 size = E usage 365×PSH

Where:

Eusage 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 size = 6500 365×4.22 =4.22[kWl 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 size =4.22×1.25=5.28[kW]

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.

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:

  • 5200/200W(per module)=26 modules. We can split them in TWO strings of 13 each
  • 5200/240W(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.