PVsyst Losses & Some Remedies
Amit Upadhyay elaborates upon the losses in a typical solar PV system with reference to the PVsyst software package.
Solar system installation
Those in the solar business are well aware of the PVsyst software – a PC software package for the study, sizing and data analysis of complete PV systems. This is used to simulate the generation potential of solar PV plants at various locations. For that we have to consider some losses also, so this article is about the various losses, which need to be considered while preparing the PVsyst file and also how to optimise these losses.
Near Shading: This is basically shading loss due to lesser inter row distances (pitch) of two consecutive solar module rows. As per PVsyst this can be further categorised into two sub-categories:
1) Irradiance losses: Due to lesser space, in between two consecutive rows some amount of irradiance in the morning and evening will not collected by solar cells so termed as irradiance loss formerly called "Linear shading losses”.
2) Electrical losses: How you form the strings on structures whether I or U, based on that "according to string" minimal amount of loss present in the system.
Depending on the tilt of the structures this can be up to 2.5% maximum.
IAM factor on global: The incidence effect (the designated term is IAM, for "Incidence Angle Modifier"), which corresponds to the decrease of the irradiance really reaching the PV cells' surface, with respect to irradiance under normal incidence, due to reflexions increasing with the incidence angle. This will be calculated by PVsyst itself as this is module dependent. I have done many PVsyst simulations and found this shall not be more than 3%.
Soiling loss factor: There is no measurement device, which can measure the soil/dust/bird droppings at site, and this is generally considered maximum up to 2%, but can be further reduced up to 0.5% if we can reduce the time cycle for module cleaning.
PV loss due to irradiance level & PV loss due to temperature: Both the losses are dependent on .PAN file of module manufacturer and also completely dependent on the site meteo data, shall be calculated by PVsyst internal simulation.
Module quality loss: This loss refers positive and negative Wp tolerances of modules. Following cases shall be considered while choosing solar modules:
1) If both tolerances are present means positive and negative (i.e., ±5Wp) then this will add loss in the system so that generation become less.
2) If only positive tolerances will be present then generation will be more so gain of max up to +0.4%.
LID – Light Induced Degradation: LID occurs when oxygen impurities in the silicon wafers react with the doped (p type) boron or gallium in the first few hours/weeks of illumination of cell. The effect can reduce cell efficiency from 2 to 4% right of the bat. Better to get the confirmation from module manufacturer for first year degradation of solar modules so as to LID loss.
Module array mismatch loss: Mismatch losses are function of production electrical uniformity and binning thereof. If module comes with bin class then surely this will be less say 1% otherwise more if electrical characteristics (Current/Voltage) are different in comparison with each other modules.
Ohmic wiring loss: As the name implies this loss is due to cables selection, representing the loss on DC side in between module to inverter through DC cables. This shall not be more than 2% @ STC, superior solar designing while selecting right size of DC cables can reduced further up to 1% depend on the shape of land and contract also.
Inverter Loss during operation: This loss completely dependent on the .OND file of inverter manufacturer. While selecting inverters through efficiency we can know the loss figure.
Auxiliary losses: As the name implies auxiliary means various loads are present in solar plant which will take power for running during day time as well as night time. This loss shall be in between 0.7% to 1% depending on the size of the plant.
System unavailability: During O&M contract this loss shall be finalised based on mutual understanding in between client and O&M company; this loss shall be in between 0.5% to 1% depend on the size of the plant.
AC ohmic loss: Representing the loss on AC side in between inverter to evacuation through AC cables. This shall not be more than 0.5% at full load.
External transformer loss: Basically this loss refers to inverter transformer loss, considered up to 1.1% maximum.
Fuse Selection Criteria of SMB
Fuse Selection Criteria of SMB (String Monitoring Box) in case of Solar System Design
In case of electrical system design anywhere as per standard practice we select fuse/breaker size with the help of the below formula:
Fuse/Breaker rating size = Current x 1.2 or 1.3 times
Suppose we selected 280Wp module, as per datasheet Isc is 8.68Amps so Selected Fuse = 8.68Amps x 1.25 = 10.85Amps so selected fuse say 12Amps
But in case of solar system design this formula many times fails to provide the correct rating of fuses or fuses burnt frequently at site in the period of summer season.
Before starting we all have to know about this fact that in case of solar modules, irradiation is proportional to module current as well as temperature is proportional to module current (minutely) also, which means when irradiance and temperature will increase than module current will also increase from its rated current.
The reasons for burning of fuse (my own practical experience):
As noted in datasheet, all electrical parameter specified at STC 25°C cell temperature and 1000W/m² irradiance, in summer season radiation will increase from 1000W/m² to 1100 to 1200W/m² also temperature will increase.
Say at 1100W/m² and 40°deg ambient temperature Isc will become from 8.68Amps to 9.62Amps
Now when inner box temperature of SMB is 65°C (fuse holder) then as per derating datasheet of say Bussmann fuse, fuse current carrying capacity will become from 12Amps to 9.5Amps at 65°C.
Particularly at this instance fuses will blow, also in summer season this will happen frequently at site.
So in case of Bussman fuses they will define selection of fuses as per below formula:
Fuse rating = Isc x 1.56 times
Fuse rating = Isc (8.68Amps) x 1.56 times (very safer side) = 13.54Amps so selected fuse say 15Amps
Conclusion: Before selecting fuse rating, solar designer has must to know about following information:
- Maximum Irradiation and ambient temperature of site in summer, and
- Selection of SMB based on internal temperature rise calculation of each component.
Amit Upadhyay is Manager – Design & Engineering, at Mahindra Susten. The article is an expression of his personal experience and not related with his current organisation, and suggestions and other views are welcome. He may be contacted by mail: firstname.lastname@example.org