Together with eleven European partners the Kassel-based research institute ISET has launched the research project PV-MIPS to develop a solar module with an integrated inverter that can feed solar electricity directly into the grid. With this new product, the partners aim to reduce the total costs of a PV system to 3 Euro per Watt.

A photovoltaic system achieves the highest output when every solar module
is continuously operated at its maximum power point. This can be reached by using module-integrated inverters, since every solar module has its own controller. Module-integrated inverters lead to higher yields especially with
solar modules that are partially shaded or aligned with different angles.

Other advantages are that the design of the photovoltaic system is more flexible and that the system can easily be expanded. In addition, costs for direct-current wiring do not apply. Within PV-MIPS, solar modules with integrated inverters are being developed that can feed solar electricity directly into the grid. The challenge is to reduce the total costs of a photovoltaic system. At the same time, lifetime and reliability of the integrated solar power inverter shall be
increased considerably. There is a growing interest in module-integrated converter concepts but the market share of inverters for module integration remains small. The currently available
alternative current (AC) module inverters are intended for use in conjunction with common photovoltaic modules.
No integrated AC modules are commercially available to date. The inverter manufacturer and the photovoltaic module manufacturer are indeed two separate entities, each delivering a product that is optimized and manufactured independently.

The most relevant factors which have prevented a wide spread of AC modules are:
• high failure rate and limited lifetime;
• high specific costs (from EUR 1/W to EUR 2.5/W),
compared with central or string inverter topologies;
• labour-intensive installation;
• low efficiency (less than 93 %).
The actual figures regarding price and efficiency must be improved by the new developments within the PV-MIPS project. The real inverter costs shall be lower than EUR 0.3/W and the targeted European efficiency is 95 % (maximum efficiency 97 %). If these aims are met, the new
inverter will be strongly positioned against the state-of-the-art devices and can compete with system topologies based on string inverters.

Approach used in PV-MIPS Project
New layouts and concepts for the photovoltaic module (e.g. large area, high-voltage module) will be designed and selected in order to optimize the match between direct current output from the module and inverter input. Newly developed semi-conductor power modules will be utilised in the photovoltaic inverter. In addition, electric connection and mechanical mounting of the modules will be considered, to complete the optimization of the whole system.
This project gathers essential knowledge, technology, as well as production and market experience from different European countries. The project consortium incorporates module and inverter manufacturers, utilities and research institutes in the field of photovoltaics and power
electronics.

The development and demonstration of module-integrated inverters within the project are related to different photovoltaic module technologies. In total three module inverters and AC module systems respectively will be demonstrated within PV-MIPS.
Due to the high market share of crystalline solar cells, a module inverter will be developed for this technology based on a multi-stage topology with isolation and a DC range that matches most photovoltaic module specifications. In contrast, the share of thin-film modules is expected to increase. Thus, based on photovoltaic modules in CIS technology, two alternative solutions are under development.

An AC module system developed by the industrial partners consists of optimized CIS modules with a maximum power point voltage of 80 V and an adapted inverter. Compared with crystalline photovoltaic modules, one significant advantage of CIS is the capability to design modules with a high DC level of several hundred volts. This property is exploited in a third development line using an inverter, which takes advantage of a high input voltage.

Results
Within PV-MIPS, a transformerless three-phase inverter for the integration into high voltage CIS modules has been developed. The chosen topology – a PWM Current Source Inverter – features a single-stage power conversion system that feeds directly into the grid. This topology has been used for the first time in a low-power application such as the 250 W photovoltaic module. Due to the three-phase grid connection to the DC link, energy storage can be significantly reduced. No electrolytic capacitors are necessary which are generally highly temperature-sensitive and thus have a short lifetime. A highly efficient laboratory prototype of a compact low-power (250 W) inverter has been implemented and tested. A maximum conversion efficiency of more than 97 % has been achieved. High efficiency is not only a selling point, but it also contributes to improved
reliability of inverters as the thermal stress is reduced. In order to show the features of AC module systems and module inverters, several demonstration plants have been set up, which will be extended in the future.

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