Download mpp solar tracker: the effect of module temperature and insolation

MPP SOLAR TRACKER: THE EFFECT OF MODULE TEMPERATURE AND
INSOLATION
To ensure that a photovoltaic system permanently delivers maximum power, the inverter's MPP tracker must
constantly determine the optimum operating point of the PV generator. The generator characteristic and hence
the MPP (Maximum Power Point) of the photovoltaic system changes over the course of a day.
The MPP tracker has to react to these changes in the current-voltage characteristic. Factors that influence the
current-voltage characteristic include different module temperatures and insolation intensities. This article
describes the effects that these factors have on the current-voltage characteristic. It also explains the
requirements that an MPP tracker must satisfy in order to ensure that the photovoltaic system generates the
maximum yield.
Static MPP versus dynamic MPP
In central Europe, around half the annual energy yield is generated under changing environmental conditions.
This suggests that an inverter's MPP tracker should exhibit very rapid, dynamic operating behaviour. However,
in many cases it is sufficient if the inverter can guarantee very good static operating behaviour. This is
explained below.
Different module temperatures
The MPP voltage of a solar module varies greatly with module temperature. At the same time the temperature
coefficients of a solar module can differ considerably. In the case of crystalline solar modules, the coefficient is
between minus 0.3 percent per °C and minus 0.55 percent per °C, and in the case of thin-film modules between
minus 0.2 percent per °C and minus 0.5 percent per °C.
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Figure 1 shows the rise in module temperature with a sudden change in insolation intensity from ~300 Watt/m
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to ~800 Watt/m . The module temperature changes far more slowly than insolation – a stable temperature is
reached after approximately 30 minutes of constant insolation.
The MPP voltage of a solar module under STC (standard test conditions) is virtually insolation-independent
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above an insolation intensity of ~300 Watt/m (see Figure 2). Consequently, the MPP voltage of the PV
generator, after an abrupt rise in insolation, only changes with the module temperature. Therefore in this case
the inverter's MPP tracker does not have to exhibit rapid, dynamic operating behaviour.
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Figure 1: Temperature effect: change in module temperature with a sudden change in insolation. After constant
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insolation of ~800 Watt/m it takes approx. 30 minutes for a roughly stable module temperature to become
established.
Figure 2: Current-voltage characteristics of a crystalline solar module at different insolation intensities under
standard test conditions (STC).
Varying insolation intensity
Here it is necessary to distinguish between a slow and a rapid change in insolation intensity. One must also
consider the insolation range within which these changes take place, i.e. with low or high insolation. On a sunny
day, the insolation intensity changes relatively slowly. Although, depending on the location, it can reach values
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of up to ~1,000 Watt/m , abrupt changes are unlikely.
As explained above and illustrated in Figure 2, the MPP voltage remains virtually constant above an insolation
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of ~300 Watt/m . Therefore, above this insolation value it is sufficient if the inverter exhibits good static
operating behaviour, as the MPP voltage changes only slowly with module temperature (see Figure 1). With an
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insolation strength of less than 300 Watt/m , greater demands are placed on the MPP tracker because the MPP
voltages vary more in this range (see Figure 2). With low insolation intensities, the inverter must exhibit dynamic
operating behaviour. Figure 3 shows the periods in which the inverter has to exhibit static or dynamic operating
behaviour on a sunny day.
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Figure 3: Insolation profile on a sunny day: above an insolation intensity of 300 Watt/m static inverter operating
behaviour is sufficient. In the early part of the day and the evening, dynamic operating behaviour is needed.
On days with partial cloud, insolation can change quickly, resulting in high insolation peaks as shown in Figure
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4. With rapid insolation changes in the range of ~300 Watt/m to 1,000 Watt/m , the MPP voltages again
change only slightly. Therefore, with abrupt changes in this insolation range, very good static inverter operating
behaviour is likewise sufficient (see Figure 4).
However, larger MPP voltage changes may also occur. With an unusual, sudden insolation change from, say,
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200 Watt/m to 1,000 Watt/m , rapid, dynamic inverter operating behaviour is required.
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Figure 4: Insolation profile on a day with partial cloud: with abrupt insolation variations in the insolation range
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from ~300 Watt/m to 1,000 Watt/m , static inverter operating behaviour is generally sufficient. However, with
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insolation jumps from low insolation values (<300 Watt/m ) to insolation values >300 Watt/m , dynamic inverter
operating behaviour is necessary.
On overcast days, the MPP voltage changes only slightly, as the insolation on the solar modules remains
relatively constant on such days – static inverter operating behaviour is sufficient.
Basically it can be said that, with varying insolation intensities, both static and dynamic inverter operating
behaviour is important. It is apparent that static operating behaviour is sufficient most of the time, whereas
rapid, dynamic operating behaviour is needed only occasionally. Rapid, dynamic operating behaviour is
required mainly at low power.
Summary
Although the PV generator is frequently exposed to dynamic environmental conditions, static inverter operating
behaviour is often sufficient. Nevertheless, rapid, dynamic operating behaviour is also sometimes needed. For a
PV system to generate the highest possible yield, a good combination of static and dynamic inverter operating
behaviour is required.
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Photographs: Fronius International GmbH, reproduction free of charge.
About Fronius International GmbH
Fronius International GmbH is an Austrian company with headquarters in Pettenbach and other sites in Wels,
Thalheim and Sattledt. With 3,250 employees worldwide, the company is active in the fields of battery charging
systems, welding technology and solar electronics. Around 95% of its products are exported through 17
international Fronius subsidiaries and sales partners/representatives in over 60 countries. In financial year
2010, the company generated a total turnover of 499 million euros. With its outstanding products and services
and 737 active patents, Fronius is a global technology leader. 392 employees work in research and
development.
Enquiries:
Editor: Dietmar Staudacher, +43 (664) 8502427, [email protected], Froniusplatz 1, 4600 Wels,
Austria.
Trade press: Andrea Schartner, +43 (664) 88536765, [email protected], Froniusplatz 1, 4600
Wels, Austria.
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