Optimal PV-to-Inverter Sizing Ratio
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Properly matching PV and inverter capacities is a balancing act, but one that can improve grid-connected system performance. In his article “Sizing of Grid-Connected Photovoltaic Systems,” Jayanta Mondol of the University of Ulster states: “Optimal sizing depends on local climate, surface orientation and inclination, inverter performance, and the PV/inverter cost ratio.” Mondol recommends examining the PV-to-inverter sizing ratio (Rs) “in terms of energetic performance—the annual PV system output per PV rated capacity—and economic performance, the annual PV system output per total capital cost.” Arguably, inverter reliability and availability are also important factors.
Many inverter manufacturers have blanket sizing ratio assumptions embedded in their string sizing programs. But climate variations in North America are quite extreme. In your experience, how can integrators best define the optimal sizing ratio for their location based on local climate conditions or array mounting? What impact does falling module prices have on this relationship? What impact does equipment specification have? With all of these variables, what are the most important considerations for designers, and what, if any, rules of thumb do you recommend?
Sizing Ratio Should Optimize Specific Yield
Allan Gregg / Satcon / Boston, MA / satcon.com
The goal of the PV system designer is almost always to configure the system components for maximum energy throughput from the PV panels to the grid, while optimizing specific yield. Specific yield is calculated by dividing kilowatt-hours delivered over time by the peak installed kilowatts. This calculation takes into account all system losses. The optimal PV-to-inverter sizing ratio according to specific yield will vary from system to system, based on the designers’ allowances for the various derate factors. This design process can be performed based on educated estimates, detailed calculations or performance modeling. However it is done, the best performance can be achieved only by sizing the peak capacity of the array larger than the maximum power rating of the inverter.
All PV panels are assigned a peak power rating at STC, which is a light energy level of 1,000 W per square meter, a cell temperature of 25°C and an air mass of 1.5. Unfortunately, these conditions rarely, if ever, exist simultaneously. Experienced system designers know this, and they try to account for the various other factors that negatively impact the output of any PV array. These factors include the following:
- Module soiling. Except during and immediately following a rain shower, a fine layer of dust covers the surface of an array, even one that appears clean.
- Module power tolerance or mismatch. PV modules often have a +5%/-5% power tolerance. The maximum power operating points of paralleled modules do not exactly match. Modules are forced, due to parallel electrical connections, to operate at a less-than-optimum voltage.
- Elevated cell temperature. Power output is inversely proportional to cell temperature. Even on a cool day, the sun warms cells to temperatures above 25°C.
- Low irradiance conditions or nonoptimal tilt angles or both. Weather conditions or array tilt routinely result in plane of array light intensities less than 1,000 W per square meter.
- Module aging. Silicon-based PV modules generally degrade by a little less than 1% per year.
- DC wiring losses.
All of these factors work to reduce the available array power. Consequently, even before other system losses are accounted for—such as inverter or transformer losses—the output of a PV array is always derated by some factor. This factor is the basic premise on which a system designer bases the calculation that determines the sizing ratio between the array capacity and the inverter capacity. In order to utilize the system components to their full capacity, and to optimize specific yield, the peak array power should always be greater than the inverter capacity. The question is how much greater.
It is common in the industry to oversize the PV array by using a PVto- inverter sizing ratio of around 1.15. After all, a well-designed system will typically have end-to-end system losses of about 15%–16%. Oversizing the array ensures that the inverter is driven to its maximum output, at least during the best sun hours of the day. System designers who are looking at a 20-year design life for the system will usually size the array-to-inverter using a 1.2 to 1.25 ratio. Some PV system integrators even routinely use a 1.3 ratio.