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August 13, 2012

The Right Device for a High Yield

David Bassie
Product Manager, Solar Division, Mastervolt

Power Channels: Energy Efficiency, Renewable Energy

The choice of inverter has a major bearing on a PV installation’s yield. But efficiency is not the only key factor to consider.

When designing a PV installation, the aim is to maximize energy production and avoid power losses. Some of the possible losses are attributable to the conditions in which the solar modules are operated. For example, rated model output is determined at a temperature of 25°C. In practice, however, modules are generally exposed to higher temperatures than that, which reduces the actual power output. Lower yields may also be caused by reflections and by modules with slightly different capacities being combined (such losses are also called “mismatch losses”).

A further proportion of lost energy is influenced by the inverter. When choosing a suitable inverter, it is important to compare different models, but not just in terms of EU efficiency. Several other factors play an important role in terms of the yields that can be expected. Mastervolt, a Dutch inverter manufacturer, has carried out a study comparing the yields of a number of PV systems in the same region. The study found differences of up to – and in some cases exceeding – 10%. Selected systems were analyzed to determine which factors influence yield. Eleven causes of energy loss were identified, seven of which could be minimized by improving the inverter. Table 1 and figure 1 provide an overview.

Cause of energy loss	Influenced by	Annual loss of energy	Annual loss of operating hours
Module mismatch	Solar module	1 – 2%
Reflection	Solar module	2 – 3%
Module temperature	Solar module	7 – 11%
Connection losses	Cable, connectors and inverter (system voltage)	1 – 2%
Downtime	All system- components	0 – 2%	85h
Conversion efficiency	Inverter	3 – 6%
MPPT efficiency	Inverter	0.2 – 2%
Late startup / early shutdown	Inverter	0 – 4%	200h
Production hours lost due to grid events	Inverter, grid	0 – 4%	175h
Inverter temperature effects	Inverter, installation location	0.5 – 1.5%
Shading, soiling of PV array	Inverter, environment, maintenance	0 – 6%
*Total system losses ()**		Min. 15% - typ. 25%	Max. 460 h/year

Table 1 – Causes of loss and how they influence total yield.

Figure 1. Typical system losses by category

Based on these results, the development team of Mastervolt has been following a number of general design principles aimed at avoiding output losses. These principles also form the basis of all new Mastervolt inverters:

High system voltage

One to two percent of the available solar power is lost in the cabling and connectors. The extent of these losses very much depends on the amperage: a doubling of the amperage results in a quadrupling of the losses.

One way of minimizing such losses it to connect one long string with the inverter instead of two short strings. This halves the amperage and doubles the voltage. Choosing inverters with a high working voltage can thus reduce cable losses by 75%. For this reason, Mastervolt has increased the working voltage of its devices to as much as 1000V.

Maximum efficiency at high insolation levels

Power conversion efficiency is by far the best-known variable influencing a PV installation’s energy production. The efficiency measured at various levels of insolation is known as weighted EU efficiency.

EU efficiency ratings have had a major influence on the development of many modern inverters. They give a high weighting to the influence of low and medium insolation values and a low weighting to the influence of high insolation values. This means that most inverters reach their maximum efficiency at low to medium insolation values.

However, recent studies have shown that PV installations actually produce most energy when insolation is at its highest (Figure 2). To maximize energy yield in the field, inverters should therefore reach high efficiencies when insolation is strong.

Figure 2. “EU efficiency” weighting factors versus actual yield in Freiburg

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