ENERGYHARVESTING
Figure 1: The
conventional DC string architecture in a solar PV system
58
Also, dust and dirt that build up unevenly on modules causes some modules or even a few cells of modules to see less solar energy than other modules. Because the modules are connected in series the whole system only performs at the level of the poorest performer in the string so a shadowed module limits the whole system and the higher potential energy of the other modules that are not shadowed is wasted. Under these real world conditions the penalty to the potential for energy harvest is severe. It is not uncommon for a system that has relatively few of its cells obstructed to perform at less than half its peak power capability and even not perform at all if such shadowing occurs on several modules in the system.
Clearly, if the weakest modules have such a dramatic affect on PV solar system efficiency, it is important that modules are closely matched before installation. This adds to both manufacturing and installation costs. In addition, PV systems based on conventional architecture can only be installed on one plane on a roof, with the modules facing in one direction. If multiple planes are used, the whole system output is limited by the performance of the modules that receive the lowest level of light.
Two further disadvantages of conventional PV
Using DC-DC optimizers maximizes the power harvested from each module by enabling per-module maximum power point tracking (MPPT). MPPT is an electronic technique that varies the electrical operating point of each module in order to extract the maximum available power from it
installations lie in the central string inverter itself. Firstly, it is a central point of failure. When it fails, all power from the solar installation is lost. Secondly, although very efficient, perhaps converting up to 99% of the available power from DC to AC, string inverters are large units that need to be located in protected environments indoors. Even then, most manufacturers of these inverters will only warrant the products for 5 or 10 years. In practice, a string inverter usually needs to be replaced at least once during the life of a PV solar installation. String inverters are big, expensive units. PV modules, on the other hand, typically perform well for 25 years.
A partial solution – the DC-DC “optimizer”
Installing intelligent DC-DC converters behind each module, then feeding the DC voltage to the central inverter, overcomes some of the problems described above. Figure 2 illustrates the architecture.
Using DC-DC optimizers maximizes the power harvested from each module by enabling per- module maximum power point tracking (MPPT). MPPT is an electronic technique that varies the electrical operating point of each module in order to extract the maximum available power from it. Using this architecture, any degradation in the performance of a module, due to clouds, shadows or other obstructions, does not affect the performance of other modules and has much less affect on the power harvested from the system as a whole. Installers don’t need to match adjacent panels for best output and modules can be installed on any available roof space – they don’t need to be on a single plane, as they do with a conventional DC string installation. In addition, the system can be monitored on a per-module basis, making it easier to identify the exact location of any problems that might occur over the life of the installation.
Despite the benefits, the disadvantages of this architecture are significant. It adds around $200 to the cost of every module without eliminating the most expensive and weakest link in conventional PV solar strings – the central inverter. Furthermore, there is still high voltage DC to deal with, requiring specialist skills and equipment, with associated higher costs. What’s more, the wiring and installation of these optimizers adds complexity and cost.
The way forward – micro-inverters
All of the advantages of DC-DC optimizers, but none of the disadvantages, can be realized by using micro-inverters. One micro-inverter is built
www.solar-pv-management.com Issue II 2010
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