Sizing of grid-connected photovoltaic systems

Grid-connected photovoltaic (PV) systems feed electricity directly to the electrical network, operating parallel to the conventional electric source. The simplest grid-connected system, such one with low-voltage for residential use, contains a PV array and an inverter unit. In high-voltage applications, the system requires transformers and appropriate power switching and protection devices. Grid-connected PV systems generate clean electricity near the point of use, without transmission and distribution losses or the need for batteries. Their performance depends on local climate, the orientation and inclination of the PV array, and inverter performance.

The output of a grid-connected PV system depends on the PV/inverter sizing ratio (R_s)¹, defined as the ratio of PV array capacity at standard test conditions to the inverter's rated input capacity. Properly matching PV and inverter rated capacities improves grid-connected system performance. Optimal sizing depends on local climate, surface orientation and inclination, inverter performance, and the PV/inverter cost ratio (T). Under low insolation (incident solar power), a PV array generates power below its rated capacity, leading to inverter operation at partial load. Inverter efficiency drops with part-load operation: it also becomes sub-optimal when a significantly undersized inverter is made to operate mainly in conditions of overload, which result in energy loss. Figure 1 illustrates inverter efficiency under both overload and partial-load operation.

Using a validated² TRNSYS³ simulation model, we studied the effects of PV orientation, inclination, inverter characteristics, insolation, and T on R_s. Parameters of a grid-connected PV system located in Northern Ireland⁴ supplied the inputs.⁵ The optimum PV/inverter sizing ratio can be examined in terms of energetic performance (C_E)—the annual total PV system output per PV rated capacity—and economic performance(C_C), the annual total PV system output per total capital cost.⁵ For different sizing ratios, C_E and C_C were normalised with respect to the corresponding maximum values, with the resulting normalised values denoted as C_E,N and C_C,N, respectively. The optimum sizing ratio is determined when the values of C_E,N and C_C,N are equal to one. PV systems with three sun tracking strategies were also considered.

The optimum sizing ratios for 0°, 45°, and 90° surface slopes are 1.4, 1.2, and 1.5, respectively (Figure 2). The incident insolation on a vertical or a horizontal surface is lower than on a 45° tilted surface, so vertical and horizontal PV arrays operate below their rated capacities. A lower value of R_s for a 45° tilted surface therefore improves inverter performance at partial load operation. Figure 3 shows the variation of C_E,N as a function of PV surface slope and sizing ratio for three surface azimuth angles.

When T decreases, maximum system performance results in lower R_s (Figure 4). That means when the inverter cost increases relative to the PV cost, an undersized inverter would optimise the system economic performance. When T increases, the system energy loss associated with an undersized inverter is not offset by the lower system cost. R_C,max for tracking systems varies from 1.1–1.6 for cost ratios between one and 14.

Figure 5 shows R_C,max as a function of T for five selected European locations. In Almeria, one of the high insolation sites, R_C,max varies from 1.1–1.5 when T ranges from one to 14, while in London, a low insolation site, the variation is from 1.8–1.2.

Under the current market scenario, for a high efficiency inverter system, PV/inverter sizing ratio for high and low insolation locations should be within 1.1–1.2 and 1.3–1.4, respectively. For a low efficiency inverter system, the optimum sizing should be within 1.2–1.3 (high insolation) and 1.4–1.5 (low insolation). Optimum PV system economic performance for an undersized inverter is achieved when the relative cost of the inverter is higher than the cost of the PV. The optimum sizing ratio is less influenced by insolation conditions when inverter cost is relatively much lower than PV cost.