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The development of a new more efficient grid connected PV module

Final Report Summary - OPTISUN (The Developement of a New More Efficient Grid Connected PV Module)

The goal of the project was the development of a more efficient and easy-to-install photovoltaic (PV) module with subdivided solar cells, back light reflector and a solar cell integrated micro inverter to increase the efficiency of the solar cell. In addition the solar cells would be parallel connected in order to control as few cells as possible to reduce the negative effects of environmental and climate influences.

The following four new technologies were developed during the project for the development of a new type of PV cell with a higher efficiency:
- Subdivided solar cells. The Si-cells should be divided into eight subparts with the same current output.
- Solar cell integrated micro inverter. The development of a micro inverter for each unit that convert the voltage from 3 to 220 V.
- Parallel connection of groups of solar cells.
- Back light reflector. The development of a back-light module to utilize the empty space between the cells and to transfer the light to the rear side of the solar cell to generate additional solar power.

The first work package aimed at achieving an enhanced scientific understanding of methods for subdividing solar cell and their effects on solar cell efficiency. The following sub-tasks were also included in this work package:
- Separate the sub-cells completely, by laser dicing.
- Separate the sub-cells without separating the wafer by trench formation realized by laser-ablation and etch isolation trenches.
- Use inkjet printing of conductive paste as interconnection principle.
- Collated knowledge design and produce subdivided solar cell with optimal performance based on laboratory experiments to be used in subsequent development.

The objective of the third work package was the development of backlight module. As far as light transmission materials are regarded the objective was to select a material for the light transmission element of the backlight module that would provide and maintain optimum light transmission efficiency during use. This objective included the following sub-tasks:
- Use developed cells and calculate the wavelengths of light that are most crucial to power generation. Identify materials that have high transmission coefficients for these wavelengths of light to optimize the match between photovoltaic cell and transmissive material.
- Consider the environmental, chemical and thermal conditions that can be experienced by the component and select a material that will retain its light transmission properties and other required properties for the expected service life of the component.

Regarding the reflector development the objective was to develop a system for reflecting the light onto the photovoltaic cells. This objective included the following sub-tasks:
- Select a coating that would reflect the maximum amount of light from the tranmissive material onto the photovoltaic cells, considering that the reflecting surface would be in contact with the trensmissive material.
- Develop a method for the application for the selected coating, ensuring that the reflecting surface retained its highly reflective qualities.

Regarding the design of the backlight reflector the objective was to design a backlight system for use with a single photovoltaic cell module and included the following sub-tasks:
- Design the light collection portion of the backlight module to capture the maximum amount of incident light that would otherwise be unavailable for conversion into power, taking the environmental conditions and effects of dirt and water into consideration.
- Design the arrangement of photovoltaic cell module, transmissive material and reflective surfaces to facilitate maximum light delivery to the cells, taking the environmental conditions and effects of dirt and water into consideration.

The development of a prototype backlight module included the following sub-tasks:
- Design and manufacture an injection moulding tool to produce the transmission component.
- Apply the reflective coating to the required surfaces of the transmissive material to produce surfaces that reflect at least 90% of the transmitted light.
- Assemble a prototype backlight module to enable further testing of its properties.

In order to test the backlight module efficiency the following sub-tasks had to be performed:
- Perform simultaneous comparative tests between the developed systems using the backlight module and the same photovoltaic system without the backlight module to access the improved efficiency under real external conditions for periods up to 4 days.
- Optimise backlight module to meet target operational performance of 1-2% improvement in efficiency without overheating the micro inverter.

The objective of the first task of the fourth work package (WP4-integration and industrial trials), was the integration bonding of micro-inverter and subdivided solar cells. This task had the following sub-tasks:
- Investigate methods of integration based on adhesives and soldering interconnection and select most suitable method.
- Develop thermal mechanical modeling of the inverter / solar cell interconnection structure to predict the reliability of the bonding.
- Optimise the interconnection process between the inverter and the solar cell with the total module interconnection system.
- Construct prototype module on lab-scale with the chosen interconnection structure to prove its performance and reliability.

Regarding the construction of prototype PV module the objective was to develop schematic concept designs integrating, subdivided solar cells, cell integrated micro inverter and backlight module to form a fully functioning PV system prototype. This included the following sub-tasks:

Manufacturing concept designs would be prepared based on a detailed understanding of the process developed in the initial work packages. An integrated OPTISUN system would be constructed integrating the subdivided solar cells, cell integrated micro inverter and backlight module for real applications and industrial trials. Finally, regarding the subdivided solar cells it was decided to use laser jet to subdivide the cells. Regarding the development of backlight reflector the following conclusions were reached:

The EU OPTISUN project demonstrated that the use of the backside of the PV cell and so more efficient use of Si/efficiency of the solar cell could be obtained by a back light module.
- Optical studies demonstrated that a dense back-light module has a higher optical efficiency than a hollow back light module. Based on the research in OPTISUN the hollow back-light modules were selected for the high efficiency PV module of OPTISUN.
- Al was selected as reflector material in the back-light module, because it was relatively cheap, easy to deposit on plastic module by PVD and had a high reflectivity.
- During the project the one-unit prototype hollow back light module was scaled up to a four-unit prototype hollow back light module.

Regarding the integration (serial/parallel connection), the segmentation of the round cells led to the mechanical need of a hole in the middle of the cell. This hole was used to lead the collected current of each individual cell segment through an interconnection foil to the micro inverter.

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