PV system performance parameters

The performance of the various PV systems is usually compared using their specific yield and performance ratio parameters. The specific yield (Yf) is the energy output (E) divided by the rated power (P_max) of the installed PV array (Equation 3.3). It defines the number of hours that the PV array needs to operate at its maximum power in order to provide the same amount of energy and is often expressed as the annual energy output per kW. The units are hours or kWh/kW, with the latter to be more preferable because it describes the quantities that are used to define the parameter. Since the Y_f normalizes the energy produced with respect to the system size, this parameter is used to compare the produced energy of PV systems with different sizes, designs, or technologies. Furthermore, Y_f is dependent on the solar resource; it varies in accordance with the irradiation. Hence, if the comparison is made for different locations or time periods, it will not be accurate because solar irradiation is varying [59, 60].

Some performance studies in European climates have shown that the different PV technologies have similar specific yield within an experimental error of ±5% [57].

𝑌𝑌

=

𝑚𝑚𝑚𝑚𝑚𝑚 (kWh/kW) or (hours) (3.3)

According to Sutterluetiet et al, energy yields can be different due to technical and commercial reasons and the technical way to ensure a high energy yield is to optimize the combination of the main loss factors as described in Table 3.1 [58].

Table 3.1: Key parameters for maximising energy yield kWh/kW [58]

Parameter Comments

Pmax nominal / Pmax nameplate High from positive binning tolerances from manufacturers

Site selection High insolation site (kWh/m²/y)

Good array orientation Tilt near latitude towards equator for best yields

Low Tmodule with proper ventilation From better thermal module design and/or free ventilation

Minimal shadowing

Try for no shading in spring to autumn day hours, if impossible string array to minimize total loss

Minimise soiling but compare the cost of cleaning and possible damage vs. lost energy yield

Electrical parameters

Normalized ISC Low dirt value, good array coating

Normalized R shunt (nRSH) Good high R shunt will minimize losses at low light levels nRSH>90%

Normalized R series (nRS) Good low R series will minimize losses at high light levels nR_S>85%

Normalized VOC Good temperature VOC coefficient, low Tmodule

Spectral correction Maximise absorption of each junction and match multi-junctions for best site specific yield

Other Proper Monitoring equipment and field

performance validation

The performance ratio (PR) is the final PV system yield (Y_f) divided by the reference yield (Y_r) (Equation 3.4), where Y_r is the system output for an ideal system and its numerical value is equal to the PV total in-plane irradiance divided by the reference irradiance. PR does not indicate the solar resource variations because of its definition and it is a dimensionless value. It describes the overall effect of system losses on the rated output due to the inverter inefficiency, wiring mismatch and general losses included in the system conversion efficiency. It also includes the losses from the PV module temperature, the partial use of irradiance due to the reflection from the module front surface, the soiling or the snow on the modules, the system downtime, and component failures [59].

𝑃𝑃𝑃𝑃 =

𝑟𝑟 (dimensionless) (3.4)

These loss mechanisms can be divided into two categories: module technology dependent and balance of system (BOS) dependent. The module technology could include losses based on the actual / nameplate power (P_max) ratio, stability (particularly for thin film technologies), module mismatch, and high module temperature effects. The BOS losses could be due to inverter downtime and low light level performance, wiring losses, shading, and soiling losses [61]. Usually, the system losses cannot “be differentiated from poor module characteristics such as degradation or fall off at low light levels, high temperatures or diffuse light unless there is a much more detailed analysis of the performance” [62]. However, the PR values are usually referred to a monthly or to an annual base. In case they are calculated for shorter periods, like weekly or daily base, they can also contribute to the identification of losses due to component failures. Moreover, due to the losses from the PV module temperature, PR values are usually higher in winter than in summer. In addition, if the PV module soiling is seasonal, it may also have an impact to the PR values from summer to winter.

Generally, if the yearly values of the PR are continually decreasing, this indicates a permanent loss in the performance of the system (i.e. degradation). In this case, the system may require some technical changes in order to solve the issues that have appeared [59].

As was discussed, the PR and the Y_f are important parameters for the PV system performance validation. However, there are uncertainties associated with the variability of modules from production lines, field measurements (especially irradiance) and the calibration process that the module manufacturer has used. Moreover, these parameters cannot specify the reason behind a performance change. For example, when the PV current drops, it can be due to module mismatch or by overall reduction of shunt resistance etc. [63]. Further, if a string underperforms on a large PV system, it will bring down the average energy output, and by averaging the energy, the yield in high irradiation levels is underestimated [61]. Hence, it would be appropriate to account for the aforementioned facts and clearly state the conditions for performance measurements and/or calculations, when PV performance results are expressed.