By Frank Rosenkranz
As fossil fuels are becoming more scarce and expensive, the eyes of the world turn to renewable energy. Carbon-free technologies such as Photovoltaic (PV) power installations are booming. This translates into tougher requirements for PV components: While low efficiency or panel failure might be a nuisance with small domestic installations, they are simply unacceptable in large-scale power generation. With this in mind, reliability, electric efficiency, high quality standards based on automated production, product traceability, global logistics and local field service are more important than ever.
At the same time PV technology is still developing. Meanwhile thin-film PV panels offer an attractive way of converting solar light into electricity. The downside of thin-film panels is their lower level of efficiency when compared to crystalline silicon panels. Thin-film panels range from 6% to 12% efficiency, while most crystalline panels yield an average of 15% efficiency. As a result, it is imperative to prevent any deterioration in efficiency during operation. This was the starting point for Tyco Electronics’ second generation PV interconnection system, which has been in development for a decade.
Attention to Detail
Pioneering solar panel installations were often equipped with connector and cable solutions taken from other field applications. While this offered an initially practical solution for the early days, the specific operating conditions of PV installations presented a unique set of challenges. Solar connectors are exposed to harsh environmental influences. Over an expected panel lifetime of 25 plus years they must withstand temperature extremes from -20 up to 80. Such wide and cyclical temperature changes cause relative movement between the connector pin and socket, which results in fretting corrosion. If the connector pins are made from tin--ass is sometimes the case--the development of a tin dioxide layer (SnO2) increases contact resistance--negatively impacting panel efficiency and frequently resulting in total panel failure.
In fact, the requirements of solar panel interconnection are so specific that the Tyco Electronics PV research and development staff initially drew a blank when they first searched the corporation--nearly 500,000-strong product base looking for a suitable product solution. The closest the experts could come to a solution fit for PV was a single-pole automotive connector. Unfortunately, this connector was designed for a wire gauge of just 0.75 mm2, was fitted with a cable featuring only single insulation and was designed for low amp levels. By contrast, PV panels require up to 6 mm2 cable wire gauge and double insulation to cope with up to 25 amps of current and hostile ambient conditions. The cable required for a PV application proved much too voluminous to be fitted to the automotive connector’s terminal. Add to that the flat profile of the solar module cable-out and there was no match available from the company’s existing product line-up at that time. Tyco Electronics engineers decided they needed to draw on their expertise and experience to design a new product to address these challenges.
The results of their effort are reflected in the PV-specific SOLARLOK interconnection system’s details. Instead of using tin contacts, the IP 67 connector range is equipped with silver-coated contacts. Silver oxide (Ag2O) that results from fretting corrosion is an electric conductor--unlike tin dioxide, which increases contact resistance.
Junction box designed for mounting on the rear of a crystalline PV module
Open System Architecture
As the complete PV array interconnection system, as shown in Figure 1, is exposed to harsh environmental conditions, there is little point in optimizing single components. If a solar power station’s initial efficiency level is to be maintained over its entire lifetime, a consistent high-performance interconnection system like the SOLARLOK interconnection system is needed. It comprises:
● A scalable junction box platform with 2 to 6 rails and blocking diodes for serial or parallel interconnection of crystalline and thin-film PV panels;
● Cable couplers (male and female);
● T-branch connectors;
● DC-AC inverter receptacles;
● IEC 60228 Class 5 cable (flexible stranded tin-plated copper), certified by the UL (Underwriters Laboratories) and the German Technical Control Board, TV;
● Main switch with 60 A current carrying capacity and integrated over-voltage protection to separate the complete PV array from the inverter; and,
● Application tooling.
The complete system is designed for fast installation during panel production, with set-up performed on-site. Polarity-keyed housings, fully shrouded contacts, reliable high cycle life, and squeeze to release connections are the result of vast connector technology expertise and a tradition of compliance with strict industry standards. Leading German panel manufacturers, for instance, use the SOLARLOK interconnection system as preferred technology because it was specifically developed for PV applications. During component development, intense validation work was carried out at the Tyco Electronics testing facilities at Bensheim, Germany, to measure component performance--including performance under extreme climatic conditions.
Crimping Versus Soldered Joints
Highly refined crimping technology is an important pillar of long-term reliability. Through its global application tooling division Tyco Electronics provides tools to ensure the formation of cold-welded microjunctions during crimping. The contact used in SOLARLOK connectors originates from a proven design for industrial machinery--but has been modified to suit another complex requirement: while the front of the socket contact has to show sufficient tension, the rear must be soft enough to avoid crack formation during crimping. Tooling and crimp design are closely interdependent areas of expertise--ensuring that the integrity of crimping joints is at the level of a perfect solder joint, but without the need to X-ray each individual joint.
Figure 1. Schematic overview of a PV installation’s many electric interconnections from panel to DC-AC inverter (Source: Tyco Electronics)
Emerging thin-film PV panel technology poses another specific challenge. Because thin-film panel efficiency is lower than for crystalline silicon panels, the capital cost for the investor must be lower to help ensure adequate return on investment. But lower capital cost cannot compromise long-term reliability. To help ensure reliability over time and under extreme conditions, Tyco Electronics has developed special junction boxes for thin-film panels within a 5-year EU program during which Tyco Electronics worked closely with partners.
One key feature of the Tyco Electronics boxes is the blocking diode between the rails. This is a crucial safety element which addresses a specific property of solar cells--that is, if a cell fails, it will convert from a power supplier to a consumer--effectively draining electric energy from other cells. This, in turn, heats up the malfunctioning cell and the resulting temperature can destroy the panel. The blocking diode prevents this by isolating the malfunctioning panel section from the others. The fully automated box manufacturing process is equipped with diode testing stations.
Frank Rosenkranz is EMEA Product Manager of Alternative Energy at Tyco Electronics Corporation (www.tycoelectronics.com). Rosenkranz is also global lead for the growth and development of Tyco Electronics’s crystalline photovoltaic product portfolio. His field of study includes mechanical engineering and economy.
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