PVSyst Design Project setting Consideration

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PVSyst Design Project setting Consideration

In simulation, an analyst runs multiple scenarios to predict how a system or process performs under different conditions, and it's the basis for predictive analytics. Modeling, also known as optimization modeling, differs in that it can determine a specific, optimal or best outcome of a specific scenario.

Project Settings:

"Project settings" that will give you access to the common project parameters, namely the Albedo values, the design conditions, design limitations and interface preferences.

In PVsyst, the albedo coefficient is the fraction of global irradiation that is reflected by the ground in front of a tilted plane. The default albedo coefficient in PVsyst is 0.2. However, the albedo coefficient may need to be adjusted based on the actual situation, especially for double-sided PV modules.

Here are some albedo values for different surfaces:

Urban environment: 0.14–0.22
Grass: 0.15–0.25
Fresh grass: 0.26
Fresh snow: 0.82
Wet snow: 0.55–0.75
Dry asphalt: 0.09–0.15
Wet asphalt: 0.18
Concrete: 0.25–0.35
Red tiles: 0.33
Aluminum: 0.85
Copper: 0.74
New galvanized steel: 0.35
Very dirty galvanized steel: 0.08

Albedo values are used in PVsyst simulations to compare the performance ratios of the resulting systems. The effect of albedo can be significant, especially when snow is present.

Design Conditions

The second tab in the project parameters dialog contains the "Design Conditions"

These are only used during the sizing of your system, and they are not involved in the simulation. The "Lower temperature for Absolute Voltage Limit" in this tab is an important site-dependent value, as it is related to the safety of your system (it determines the maximum array voltage in any conditions).

Lower Temperature for Absolute Voltage Limit:

The "Lower temperature for Absolute Voltage Limit" is an important site-dependent value, as it is related to the safety of your system (it determines the maximum array voltage in any conditions). Ideally, it should be the minimum temperature ever measured during daylight at this location. You can set this temperature as lowest possible temperture from site metro file actual site minimum temp. -10°C e.i lower in mountain climates.

Winter operating temp. for Vmpp Max:

This represents the cell temperature during usual operating conditions in winter. This is used for the maximum number of modules in series, in order to stay below the Vmpp Max input voltage value of the inverter. You can set this winter temperature from site metro file actual site minimum temp.

Usual operating temperature under 1000w/m2

Standard Test Conditions are defined at 25 ºC solar cell temperature with 1000 W/m2 solar irradiance under an air mass of 1.5. TOper is an intermediate value, used sometimes in the program or report for characterising usual operating conditions (default 50°C).

Summer Operating Temperature for Vmpp Mim design:

This represents the cell temperature during usual operating conditions in summer, under full sun. This is used for the minimum number of modules in series, in order to stay above the VmppMin input value of the inverter. You can set this summer temperature from site metro file actual site maximum temp.

These conditions are a common practice, adopted by everybody. They are specified in the norm IEC TS 62738 (2018), paragraph 7.2.1. You are not advised to cheat with this evaluation, because it may affect the safety of your installation and the warranty conditions.

Other design parameters:

Here are some design parameters related to IEC in PVsyst:

Array max. voltage

The maximum allowable array voltage, which is specified by the PV modules. The IEC standard requires 1000 V. For a given PV module, you can have simultaneously an IEC certification (most of the time 1000 V), and a UL certification (usually 600V) for use in the USA.

This has nothing to do with the simulation, it is an indication of the regulation rules in different countries (UL in the US, IEC for all other countries).

muVoc value

This option allows the user to use a derate factor specified by the manufacturer.

Limit overload loss for design

This parameter allows the user to increase the limit on the acceptable loss during the year

"Limit overload loss" in solar design refers to the maximum acceptable percentage of energy loss that should occur when a solar array is slightly oversized compared to the inverter capacity, essentially setting a threshold for how much "overloading" is considered acceptable in a system to optimize energy production while minimizing potential efficiency losses due to the inverter operating near its maximum capacity; typically, this limit is set around 3% in most solar design software like PVsyst.

Key points about limit overload loss:

Purpose:

By setting a limit overload loss, designers can intentionally size a solar array slightly larger than the inverter to capture more energy during peak production periods, especially in situations where the inverter may be limited by other factors like grid connection restrictions.
Impact on efficiency:

While overloading can increase energy production, exceeding the acceptable limit can lead to significant efficiency losses as the inverter struggles to handle the excess power, potentially causing clipping and reduced output.

Considerations when setting the limit:

Climate: Locations with large temperature fluctuations may benefit from a slightly higher overload limit as panel efficiency can vary significantly depending on temperature.

System cost: Choosing a larger inverter to avoid overloading can be more expensive, so balancing cost with potential energy gains is crucial.

Grid limitations: If grid connection restrictions limit the maximum power injection, a higher overload limit might be necessary to maximize energy production within those constraints.

How to use limit overload loss in design:

Software settings:

Most solar design software like PVsyst allow users to set the "limit overload loss" parameter in the project settings, which determines the maximum acceptable percentage of energy loss due to overloading.
Interpreting results:

When designing a system, the software will calculate the potential energy loss based on the chosen overload limit, allowing the designer to adjust the array size accordingly.

Transposition Model

In PVsyst, the transposition model is a method for calculating the incident irradiance on a tilted plane from horizontal irradiance data. The transposition factor is the ratio of the incident irradiation on the plane to the horizontal irradiation. It represents the irradiance gains or losses when tilting the collector plane I.e. what you gain (or loose) when tilting the collector plane. It may be defined in hourly, daily, monthly or yearly values. The result depends namely on the diffuse irradiance.

PVsyst offers two transposition models:

Hay's model:- a classic and robust model which gives good results even when the knowledge of the diffuse irradiation is not perfect,

Perez model:- is a more sophisticated model requiring good (well measured) horizontal data

For fixed planes the difference may be of the order of 1%, depending on the tilt of course.

AC loss power reference:

In PVsyst, you can define the AC loss power reference for a project . This allows you to set the default initial value for new projects. AC wiring losses are caused by the impedance between the inverter output and the injection point. The wiring loss basic parameter is the resistance of the wires. I.e. the Length and section, as well as the metal. The percentage loss is relative to the operational power, and we can calculate that it is proportional to this power.

AC loss with Pnom is accurate as it is considered Inverter power for percentage loss calculation. PVsyst proposes 2 options: either the PNom of the inverter, or the PNom of the PV array. So that for a same loss (same resistance), the percentage parameter will be lower for the PNom of the inverters than for the PNom of the PV array.

Circumsolar Treatment

The circumsolar treatment is an option that allows the user to distinguish the circumsolar component from the diffuse and beam components. However, the circumsolar component will not produce electrical shading mismatch losses. In all usual meteorologigal data, the circumsolar contribution is accounted with the diffuse component.These improvements are not very important in usual systems with low plane tilt, and negligible in tracking systems. They become crucial in vertical bi-facial systems modelling.