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Manufacture of PV modules: Glimpse into production hall.
© Heckert Solar
Photovoltaics - optimisation of modules

Depiction of CTM analysis for a standard industrial 5-busbar module, calculated using the SmartCalc.CTM software tool. The analysis clarifies the influences of the different gain and loss mechanisms on module performance.
© Fraunhofer ISE

Depiction of optical and electrical loss mechanisms factored into the computational model for the quality assessment of PV modules.
© Fraunhofer CSP

A typical Si-based PV module; illustrates the various loss and amplification mechanisms in schematic cross-section and top view: 1) air/glass reflection loss, 2) glass absorption loss, 3) glass/encapsulation material reflection loss, 4) encapsulation material absorption loss, 5) cap./cell coupling amplification, 6) reflection on inside of glass, 7) losses at module edge and in space in between, 8) diversion intensification from inactive area, 9) shadowing losses of contacts (busbars, contact fingers), 10) light diversion of contacts, 11) ohmic losses of circuitry/contacts.
© Fraunhofer CSP

Enhancing performance of PV modules

The output of solar modules is usually less than the cumulative performance of the solar cells installed in these. Various optical and electrical loss mechanisms as well as optical gains influence the output of cells integrated into the photovoltaic module. Researchers are looking for ways to enhance the performance of modules without modifying cells. This involves combining components in a way that ensures that the achieved module output is greater than the added performance of individual cells.

Working with a total of 12 industrial partners, the Fraunhofer Institute for Solar Energy Systems ISE and the Fraunhofer Centre for Silicon Photovoltaics CSP are investigating gain and loss mechanisms from the solar cell to the photovoltaic module. Their hope is to further enhance the direction of sunlight into the solar cell and to minimise optical and electrical sources of loss in the PV module. The aim being to improve the relationship between module output and the sum of cell performance without modifying the integrated cells themselves.

This performance ratio from the cell to the module is described by the coefficient CTM (cell-to-module). The researchers hope to create higher-performance modules from cells in current industrial production. “The aim of the CTM100+ project is to produce quality modules that achieve over 100% cell output,” explains Prof. Jens Schneider from Fraunhofer CSP, the project’s coordinator. Installed cells consequently are expected to produce greater output within the module than they would individually.

The researchers have developed a technology roadmap for quality solar modules with a CTM of over 100%. They are investigating in this respect materials used in addition to the module structure. To ensure the sun’s rays reach the solar cells with as little obstruction as possible, reducing glass reflection for instance is one such avenue pursued. The researchers have produced various module materials, material combinations and module designs. These they have distinguished in terms of their technical measurements and have subsequently recorded the combined effect in optical and electrical modules. To achieve continual improvement, the two Fraunhofer institutes produced new optimised modules. The institutes reviewed the improvements attained together with an expert group comprising system builders and manufacturers of components, measurement systems and modules.

Software tool helps developers and manufacturers boost module output

A classification scheme developed at Fraunhofer ISE for measuring and calculating the individual gain and loss mechanisms was used and further refined for CTM analysis of the different cell and module designs. These calculational models were carried over into a software tool. The tool is used to quickly calculate cell-to-module losses and gains for solar modules and facilitates flexible virtual prototyping.

The software ascertains optical, electrical and geometrical losses and gains of cell output. It allows PV manufacturers to gauge the effects different materials have on module performance. This enables manufacturers to optimally combine components and to produce modules with the best possible output. The software also enables the effects of new materials and components to be assessed in advance.

The optical and electrical properties of different solar cells and module materials are contained in the system. The system comprises various module designs and solar cell types, as well as contacting and circuitry methods. It allows manufacturers of glazing products, anti-reflection coatings, embedding films, cell connectors and rear side films to first simulate conventional and new module structures on a computer and to assess material properties directly at module level.

The optimisation software has been used for instance at the Module Technology Evaluation Center of Fraunhofer ISE to produce half-cell modules with a CTM of 104% (module output 256 W) and multi-busbar modules with 306 W. PV manufacturer Heckert Solar achieved a performance enhancement for its own modules of approx. 0.5% using the software. The company intends to continue using the system to ensure continual improvement of module performance.

New computational model for predicting performance losses

Fraunhofer CSP unveiled a further refined model for the quality assessment of PV modules in April. The software makes allowances for the interaction of irradiated light and electricity, as well as heat production. It factors optical and electrical effects into the calculation of optical reflections, heat sources and energy losses in photovoltaic systems. The computational model relies in this respect on experimental data collected using standard measuring equipment. Based on an optional (solar) spectrum, it allows such losses to be reliably predicted. In addition to CTM values, it also provides information regarding heat losses occurring in the PV module.



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Project management, simulation software, module prototypes
Fraunhofer CSP

CTM optimisation software and module simulation
Fraunhofer ISE

Industrial partners

Heckert Solar
PV module manufacturer

SI Module
Solarmanufaktur Freiburg

f | solar
High-end functional layers, anti-reflection coatings

Aluminium Féron
Aluminium films and coatings

LED solar simulator

Pi4 robotics
Optical inspection and robotics

Micro- and nano-structures for trapping light

J.v.G. Thoma
System and machine builder for PV


The CTM value describes the ratio of module performance to the sum of cell output. Today’s solar modules can exceed the output of cells. They achieve in this case a CTM in excess of 100%. Such modules achieve optical gains that are higher than the optical and electrical losses. To do so, module developers must be accurate in selecting and optimally combining suitable materials and technologies.


CTM characterisation and optimisation
Information sheet of Fraunhofer CSP

CTM software of Fraunhofer ISE

Software reduces losses
Max Mittag blog article, Fraunhofer ISE

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