c06 - Electrical design VF-AL

Introduction

The BFIRST “VF-AL” skylight module is a BIPV construction and active and passive energy multifunctional element.
The VF-AL ventilated façade combines a composite PV module and an aluminium standard construction system. The concept is based on the direct integration of the composite PV module over an aluminium ventilated façade component by ASTRAWALL.
The demonstrator will be composed by 34 VF-AL modules, in a configuration of two rows of 17 modules each, placed in the free concrete space under the testing laboratories (black building).

 Electrical layout and real prototype of ventilated façade module VF-AL.

 

DC ARRAY CONFIGURATION

The solar PV fields will be configured by connecting the BIPV modules in series-parallel arrays.
The number of modules connected in series forming a string, and the number of string connected in parallel forming the complete DC array must be decided in accordance with the DC input characteristics of the chosen inverter.
A minimum DC input voltage has to be reached and a maximum input current value not exceeded, under real operating conditions. In this regards, it is essential for the system design to apply the suitable corrections for temperature, based on the correspondent voltage and current temperature coefficients expressed in the module’s datasheet.

PV array configuration

On the other hand, it is advisable to oversizing between a 15-25% the nominal power of the PV array with respect to the nominal power of the inverter, in order to optimize the potential possibilities of the inverter, which performs at the maximum of its capacity at high operating power values.

The BIPV solar fields have to be installed according to the architectural and building requirements exposed in the related Mounting and Construction Design Guidelines and the Mounting and Construction Guidelines.
Special attention has to be paid about the positioning of modules in the building: orientation and inclination should be as optimal as possible to guarantee the maximum performance.
In the same way, possible shadows should be avoided in order to loss production and preserve the health of the modules.

POWER CONDITIONING SYTEMS

DC power coming from the PV field must be conditioned in order to enable the use of the generated energy. Depending of the final use of the electricity (AC or DC supplies) and the type of connection to the electric network or load (grid-connected or stand-alone systems) a specific power system must be chosen.

Grid-connected systems

For grid-connected systems, the more common ones, a power system based on inverters, which convert direct current (DC) to alternating current (AC), is required. The type, size, efficiency and operational condition of the inverters should be chosen according to the size and characteristics of the PV field and the grid-connection requirements. All these factors will have a great influence in the system performance.
Inverters should include one or several maximum power point tracker systems, which make possible the adoption of the operating voltage which maximises the power generation. If modules are located in the building with different operating conditions (orientation, inclination and shading) between them, a distributed power managing based on the use of micro-inverters with individualised MPPT systems is highly advisable. Otherwise, if these modules were connected to the same MPPT the system would see drastically reduced its performance. The use of DC micro-converters, with individualised MPPT, together with a suitable central inverter is other option, regarding the available solutions with distributed power architecture.
On the other hand: for small sized PV systems inverters (or micro-inverters) can be connected to the low voltage panel of the building (single phase connection). In case of large systems, power should be distributed between the electrical network phases (three phase connection); this could be done by using a single inverter per phase or a three phase inverter.
For all above mentioned cases local standards and regulations have to be considered, overall in grid-connection matters, which might reduce the set of products available to carry out the power conditioning of a BIPV system.

Stand-alone systems

For stand-alone (or of-grid-connected) systems with a demand load working with AC, the criteria explained in the last point are also valid. For stand-alone systems with a demand load working with DC, a DC/DC converter use to be needed to adapt the PV field voltage to the load voltage. In both cases, battery systems and auxiliary fuel equipment are commonly included.

Single phase layout example

 

VF-AL demo-system building integrated in Spain

The electrical connection strategy of the BFIRST demo-system in Zamudio (Spain) has been decided according to the recommendations provided in this guideline.

Layout and equipment

The “VF-AL” system of the Spanish demo is composed by 34 VF-AL modules, in a configuration of two rows of 17 modules each. The total installed power is 2584 Wp, that will be transferred to a 3kW inverter “Ingecon Sun 3TL-M”, by INGETEAM. This inverter has 2 power inputs and MPPT systems. The ventilated façade system is connected to one of this inputs, the other one is used by the SL01 system, also installed in the Spanish demo-building.

PV solar fields of the BIPV systems (VF-AL & NVF01) in the Spanish demo

“Ingecon Sun 3TL-M” inverter, by INGETEAM

 
Single-line diagram of the Spanish BIPV demo-systems

 

ELECTRICAL INSTALLATION AND MONITORING SYSTEM

VF-AL BIPV modules include 1 diode incorporated, within the connection box, in order to cancel out the module if some cell brakes or a persistent punctual shadow damages some cell because the hot-spot heating effect.
VF-AL BIPV modules have plug and play connectors, which provides a secure, durable and effective electrical contact and increase the safety and simplicity of the connections during the installation works.
DC cabling transmit power from the PV field to the inverters (or micro-inverters) and AC cabling from the inverters to the grid. Both of them can be totally or partially exposed to the environment; so, they must be resistant to heat, rain, hail, ozone and UV solar light, among others. The design of the cabling layout should reduce, as much as possible, the losses associated to the internal resistance, which depends on cable length and cross-section, without incurring excessive costs.
String boxes for installing string connections and protections could be needed for large systems. Cables connecting in series the modules within a single string and cables connecting different strings in parallel give the suitable values of current and voltage required as input for the central inverters. If the system has a distributed architecture, cables coming from the micro-inverters transmit power to a bus cable, which sends the current directly to the grid connection.
String fuses or blocking diodes should be used for large PV fields, in accordance to the requirements included in the local or national regulation for electrical installations.
Switches should be also installed in order to guaranty the possibility of manually electrically isolating the PV strings or array, during the system installation, maintenance and reparation works.