H. H.C. de Moor1, N. J.C. M. van der Borg1, B. J. de Boer1
H. Oldenkamp2

1Energy Research Centre of the Netherlands, ECN
P. O. Box 1, NL 1755 ZG Petten, the Netherlands
www. ecn. nl e-mail: demoor@ecn. nl
2OKE-Services

Introduction

Building integration is at present the most important market segment for PV [1]. Surely governmental support was important, but especially in Europe it is the first major market to become cost-effective, as the solar electricity has to compete with the relatively high tariffs for private consumers.

A drawback is the usually non-optimal performance due to non-uniform illumination and failures in the modules, wiring and inverters. In this paper it is proposed to go to parallel wiring as this reduces the losses compared to series connected modules.

1 Lay-out strategy and performance loss

Building integration has a number of implications that may influence the energy performance of the PV-system, such as:

• partial shading; if only a part of the PV-array is shaded the energy loss can be over-proportional compared to the loss of incident solar energy

• multiple orientations; the PV-array may cover area’s with different orientations or more in general non-optimal orientation as the PV modules are part of the building envelope.

As the installation and maintenance is part of the building process two factors have to be taken into account in the system lay-out

• ease of installation; the cabling and mounting of the modules most be standardised to avoid costly engineering and allow roofers to install the PV system

• accessibility; the system components are often not accessible for inspection or repair requiring a lay-out that is tolerant to system failures

Partial shading and different orientations within one system are especially important in larger buildings [2], but also not negligible in family houses. These differences in the irradiance on the various PV-modules of the total PV-system, cause a loss of power of the array of modules compared to the sum of their potential individual power values. This so — called mismatch effect is the phenomenon that PV-modules connected in parallel or in series cannot operate in their individual maximum power point because their voltage (parallel) or current (series) is forced to be equal.

The maximum power point of the various interconnected modules may differ from each other due to possible individual differences in the modules, due to differences in soiling, module temperature and irradiance.

The amount of annual energy loss due to mismatch can be influenced by the electrical and geometrical layout of the PV-system. Accessibility, determined by the mechanical structure, is also of importance for the system performance. The effect of a non-functioning
module might not be restricted to that module itself but it can jeopardise the performance of other modules as well. The effect of a bad module on the amount of annual energy loss depends on the electrical layout of the PV-system; the presence of bypass diodes and the lengths of the strings (series connected modules).

Some possibilities for the electrical layout of a PV-system are the following [3]:

• Central inverter. All the modules are grouped in a number of strings. Each string consists of modules in series and the strings are connected in parallel on the central inverter.

• String inverter. This is the same a central inverter but it is based on one string only, typical in the range between 1 and 2.5 kWp.

• Multi-string inverter. This is a special application of a central inverter in which the various parallel strings are equipped with their own MPP-tracker.

• Parallel connection of single modules.

In this concept the modules are coupled to a DC-bus. The total DC-power is then converted to AC by a central DC/AC-inverter [4].

A similar approach was proposed using electromagnetically coupled modules [5].

• AC-Module inverter. In this concept each module has its own inverter [6]. The output of all parallel inverters is fed into the 230 V AC-bus.

The number of modules per string is an important design choice; on the one hand lower ohmic losses (higher voltages) for longer strings, on the other hand higher output for shorter strings (see below) and more simple safety measures due to the lower voltages. The installation is simple if all the modules are in series or all parallel. A mixed lay-out becomes complex for large systems.

An attempt was made to quantify the effects of multiple orientation (by modelling), of partial shading (by experimenting) and of accessibility (by reasoning). This has been done for realistic but arbitrarily chosen PV-systems. Therefore the results are not generally applicable but they give an indication of the order of magnitude of the addressed effects. Furthermore in the concept choice, stringing and size of inverter, more items play an important role such as the price, efficiency and reliability of the inverters. These items are not addressed in this paper.