A Designer's Guide to PVsyst: Part 05

DETAILED LOSSES IN PVSYST

INTRODUCTION

As discussed in the part one of this series, in a PV system various factors cause energy losses and these losses affect the final output. In this blog, we’re going to discuss some more about those losses.

There are different types of losses in a solar system like,

• Array losses
• DC wiring losses
• AC wiring losses
• AC losses in transformer (AC losses in transmission line & transformer)
• System losses

Losses can be defined in the PVsyst in the Detailed Losses section under main parameters. 

ARRAY LOSSES

Array losses are those losses that affect the available array output energy concerning the PV-module nominal power as quoted by the manufacturer for STC conditions.

These losses generally include:

1.     Array soiling losses

Soiling losses refer to the reduction in light reaching the solar cells due to the accumulation of dirt onto the glass cover of the modules. Its effect is uncertain and strongly depends on the weather conditions and environment around the system, as the type of soil (sand blows away, while clay sticks).

By default yearly loss of 2% is assumed in the case of manual or conventional cleaning, 0.8% in the case of robotic cleaning, and 3-4% for areas with high dust (e.g., Cement factory )

You can also set the month-wise values. Which may be more accurate but difficult to judge. Rains tend to wash the module. Summers may bring lots of sand storms leading to layers of soil, while soil may stick to the glass due to morning dew in cooler months.

2.     Thermal loss factor

Thermal losses refer to the loss in the performance of the module due to the thermal behavior of the field. The thermal loss factor is used to determine the energy loss that will occur due to the temperature difference between the nominal cell temperature at which power was specified (25 C) and the actual ambient temperature of the PV modules and the cells that further heat up due to incident sunlight. Typically, the Normal Operating Cell Temperature (NOCT) is specified by the module manufacturer and that tends to be significantly higher than the ambient.

The value of thermal loss also depends on the mounting structure of the system and is affected by airflow possible from the front and back of the modules.

PVsyst proposes default values without wind dependency for different structures.

Uc refers to the constant loss factor, and Uv refers to the wind loss factor.

It is possible to modify these values to match the site conditions, however, conservative estimates of cooling by wind are typically not used.

• For free-standing systems (with air circulation all around the collectors, like ground mount):

Uc = 29 W/m²·k,      Uv = 0 W/m²·k / m/s

• For fully insulated back (no heat exchange at the back, only one side contribution to the convicting heat exchange, like flush mount installation above sheet metal roof),

Uc = 15 W/m²·k,      Uv = 0 W/m²·k / m/s

• For semi-integrated systems(Like installation on a flat RCC roof or tilted installation above a sheet metal roof),

Uc = 20 W/m²·k,      Uv = 0 W/m²·k / m/s

3.     LID- Light-Induced Degradation

Light-Induced Degradation (LID) is a loss of performance of PV modules that happens in the very first few days of exposure to the sun. This varies from technology to technology and is provided by the manufacturer.

LID is assumed to be in between 2-2.5% for Polycrystalline, Monocrystalline, and Bi-facial technology & less than or equal to 0.5% for thin film technology depending upon the module and the manufacturer. 

4.     Module Quality Loss

This refers to the deviation in the average effective module efficiency concerning manufacturer specifications. This is also very specific to the product quality and is part of the specifications provided by the manufacturer. Lower Module Quality losses are understandably preferred but it depends on quality checks performed at the time of acceptance of the lot and installation in the field.

The value of module quality loss is dependent on the manufacturer but is generally considered to be around 0.5% to 1%.

5.     Module Mismatch losses

Mismatch loss refers to losses caused by slight differences in the electrical characteristics of the installed modules. The characteristics of each module are never rigorously identical. This mismatch leads to the string getting limited by the weakest of the cells (lowest Isc). This leads to the module mismatch loss. 

6.     String mismatch loss

This refers to the difference in the electrical parameters at the string level. In this case, the mismatch between the voltages of the strings is important as they are in parallel.

Generally, both the module and string mismatch loss are together considered to be 0.6% with a string inverter and 1.1% with a central inverter.

7.     IAM loss factor

The incidence effect called IAM ("Incidence Angle Modifier") corresponds to the decrease of the irradiance reaching the PV cell’s surface, with respect to irradiance under normal incidence. This decrease is mainly due to reflections on the glass cover, which increases with the incidence angle.

IAM losses are shown below for user-defined profiles and are provided by the module manufacturer.

WIRING LOSSES

1.     DC Wiring losses:

DC wiring losses are mainly caused by the ohmic resistance of the cabling and interconnections of the PV devices and strings. These can be estimated by summing the series resistances of each component and a simple circuit analysis of the voltage drop incurred due to the current flowing through those resistors.

As per industrial practice and from our experience, the average DC wiring losses are considered to be less than or equal to 1.5% in the case of roof-top. On the other hand, for ground mount systems it is considered to be less than or equal to 2% with a string inverter and is less than or equal to 0.8% with the central inverter. (Losses Calculation Reference: Rooftop 1 MW plant, Ground Mount 10MW single cluster)

2.     AC Wiring losses

The AC wiring losses are similarly due to the impedance between the inverter output and the injection point (or an eventual MV transformer).

The program will determine the minimum section of the wires, and only propose suitable sections if you want to increase it.  

Inversely you can also specify a loss fraction (at STC or Pnom), and according to the chosen wire section, the corresponding wire length will appear, as well as the voltage drop for the reference power.

As per industrial practice and from our experience the average AC wiring losses are considered to be less than or equal to 1.5% in the case of roof-top. On the other hand, for ground mount systems it is considered to be less than or equal to 2% with a string inverter and is less than or equal to 0.8% with a central inverter. (Losses Calculation Reference: Rooftop 1 MW plant, Ground Mount 10MW single cluster)

AC LOSSES IN TRANSMISSION LINE AND TRANSFORMER:

The losses apply in the same way as AC wiring losses. Define one or several MV/HV external transformer(s) referring to Inverter duty transformer & Power transformer. Then define the properties of the MV/HV line up to the injection point as per the power evacuation scheme. It is either an Overhead transmission line or Underground cable along with the line/Cable voltage, cross-section of wire/conductor according to current carrying capacity, and length of the line/cable.

For example, with a single MV Transformer, the MV line voltage is 20kV and the line length is 1m. The loss fraction at STC is considered 0.5%. 

Standard MV transformer losses are considered:

• 1.1% (0.1%- Iron loss / No load loss & 1.0% - Copper loss/resistive loss ) for Aluminium winding transformer.
• 0.9% (0.1%- Iron loss / No load loss & 0.8% - Copper loss/resistive loss ) for Copper winding transformer.

The below picture shows the required MV external transformer parameters. 

HV Transformer shall be defined for voltage > 100kV and Standard HV transformer losses are considered 0.5%

The below picture shows the required HV external transformer parameters. 

SYSTEM LOSSES:

1.     Unavailability of the system:

The system can be unavailable due to some maintenance or unforeseen reasons. You can define system unavailability as a fraction of time (or several days). For these failure hours, the system will be considered inactive (OFF) during the simulation.

In the general system, unavailability is considered 1.0%

2.     Auxiliary Losses:

The auxiliary’s consumption is the energy used for managing the system. This may be fans, air conditioning, electronic devices, lights, or any other energy consumption which has to be deduced from the PV-produced energy to be sold to the grid.

The auxiliary consumption depends on power consumed by equipment within the plant, generally considered less than or equal to 0.6% or 6.0W/kW Proportion to the inverter output power.  

CONCLUSION

The above losses are summarized and can be analyzed using the loss diagram that is generated by PVsyst. This loss diagram plays a key role in identifying any imperfections in the system design, so we can iterate to improve the cablings or components if possible.

Author: Jyoshna, Jr. Engineer

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