PV Modules
Polysun makes it possible to set up PV fields having each an inverter and the desired number of series or parallel-wired modules. Each PV field has a specific orientation and tilt angle as well as a field-specific assembly system.
So that different orientations and pitches may be combined each system diagram enables multiple PV fields to be defined within it. Alternatively after a module has been chosen an assistant providing an overview of the suitable inverters may be called-up by simply pressing the Wizard button.
Parameters PV Modules
The definition of a solar-module requires, as a key parameter, that a suitable PV field is retrieved from the solar-module database or independently set-up as well as that an appropriate number of modules is determined. As an option the DC nominal power and the solar module surface may be entered based on which Polysun will automatically calculate the required number of modules.
Additionally module orientation (South is 0°, East is +90°, West is -90°) and tilt angle (floor is 0°, façade is 90°) may also be defined.
Polysun also enables users to work out the yield of sun-tracking PV systems (single or two-axis solar trackers). We proceed on the assumption that the system is set-up to track the position of the sun (and not the clearest spot) and that the tracker is able to follow the entire course of the sun and not just a limited angle. For a graphical representation of this principle see chapter Tracking.
Parameter Reference Value for the Area
The power of the photovoltaic generator field can be determined through the number of modules, total nominal power or total gross area.
The number of modules is the number of PV modules of the module type selected. The total nominal power is defined as the performance of the PV modules measured in kW without deducting cable or inverter losses. The total gross area is the area of the generator field in square meters.
Temperature Effect and Rear Ventilation
The amount of energy generated by solar-modules depends both on irradiance and module temperature. Energy production increases approximately linearly with irradiance. The influence of temperature is smaller and is dependent on the type of cell-technology. A temperature increase of 10°C will cause, for example, the energy production of crystalline cells to be cut back by about 4 to 5%. Amorphous cells are practically immune to temperature swings.
In view of such temperature sensitivity an appropriate rear ventilation will result in a considerably higher yield. The software allows different types of ventilation to be set up as a parameter:
- Poor: for example a roof-integrated system with a very poor rear ventilation. With an irradiance of 1000 W/m2 the module temperature will lie at about 40°C over the air temperature.
- Medium: for example a roof-mounted system with a rear ventilation of about 10 to 20cm. With an irradiance of 1000 W/m2 the module temperature will lie at about 30°C over the air temperature.
- Good: a free-standing system with a ground clearance in excess of 20cm. With an irradiance of 1000 W/m2 the module temperature will lie at about 20°C over the air temperature.
Degradation and Soiling
PV fields are subject to degradation and soiling. Researches have shown that soiling rapidly increases in the early weeks after installation or cleaning to eventually settle to a level in the range of 2 to 6% (Source: H. Becker, W. Vassen, W. Hermann: „Reduced Output of Solar Generators due to Pollution“. Proc. 14th EU PV Conf., Barcelona, 1997). The soiling rate matches the percentage reduction in the system’s DC yield.
The degradation of solar modules as well as that of system as a whole is assumed to be a linear process. Degradation-induced yiled decreases amount to an average 0.2%/year (Source: Leitfaden Photovoltaische Anlagen; Deutsche Gesellschaft für Sonnenenergie, Landesverband Berlin Brandenburg e.V.; DGS Berlin 2005.).
Results
The yield of a solar system can be calculated by means of the H.G. Beyer model.
(Source: Beyer, H.G., Betcke, J., Drews, A., Heinemann, D., Lorenz, E., Heilscher, G., Bofinger, S., 19th European Photovolatic Solar Energy Conference & Exhibition, Paris 7.6.-11.6.2004. Identification of a General Model for the MPP Performance of PV-Modules for the Application in a Procedure for the Performance Check of Grid Connected Systems). This model relies on the following inputs:
- 3 efficiency readings for the module at different irradiance conditions.
- 3 efficiency readings for the inverter with different loads
- The installed power
- The module’s temperature coefficient
Such interpolation nodes enable an efficiency curve to be identified for the module and the inverter These curves, the installed power and the temperature coefficients allow the yield to be calculated depending on the irradiance and module temperature.
Model temperature can be calculated from ambient temperature, irradiance and gamma parameter for rear ventilation:
Module temperature = ambient temperature + gamma x irradiance/1000 W/m2
The following factors may be inferred from the resulting yield:
- Soiling (default value 2%, it can be defined in the PV field)
- Degradation (default value 0.2%, it can be defined in the PV field)
- Standard deduction for piping lossess, module mismatch and module derating: 4% + 4% x inverter load
Tracking
Tracking systems enable collectors to yield an increased heat output. The use of solar tracking systems can be more or less worthwhile depending on use and location. The closer to the equator and the larger the system and the more the use of tracking systems will be worth your while. As these locations can enjoy a higher annual irradiance start-up costs for tracking systems will be accordingly lower.
The collector characteristic dialogue box gives the user the opportunity to choose from three different types of tracking devices.
One-axis azimuth tracker
The tracker causes the collector to rotate about the vertical axis.
One-axis zenith tracker
The tracker causes the collector to rotate about the horizontal axis.
Two-axis tracker
The tracker causes the collector to rotate both about the horizontal and the vertical axis.
Simulations with sun-tracking collectors involve the calculation of computationally intensive IAM-factors. This heavily affects simulation times that may be considerably longer.