Purification and crystallization of silicon
A major research and development effort to reduce the cost of producing multi-crystalline photovoltaic silicon is currently underway.
The aim of this research is to develop new silicon purification methods and new crystallization processes for producing ingots or thin films, with the aim of reducing costs and the quantities of material used.
Because producing solar-grade silicon is very costly, reducing production costs for photovoltaic cells is a top priority.
In order to achieve this, research is being carried out in two interdependent fields: reducing the cost of manufacturing processes and increasing the conversion efficiency of cells.
INES’s Restaure Center is working on ways of optimizing manufacturing processes for photovoltaic cells in order to improve the conversion efficiency of crystalline silicon cells. Its aim is to provide companies with new technologies and to make photovoltaic electricity more competitive.
Restaure has at its disposal a 1,000-m² clean room and numerous facilities (diffusion oven, plasma-assisted CVD deposition, spectral response bench, screen printing, etc.) for processing wafers with a wide range of shapes and sizes.
It carries out research in three new fields: silicon metal cells, a junction process using amorphous silicon on silicon wafers, and nano-structured silicon. For each of these substrates, improvements are being sought at all stages of the process, from purification to ingot manufacture and wafer cutting.
Organic and innovative cells
Photovoltaic cells are made using standard materials, generally with a single junction between the materials. Due to an inherent physical limit, the use of a single junction means that the conversion efficiency of cells cannot exceed 31%. New concepts are therefore being explored in order to overcome this limit.
Using nanomaterials in photovoltaic cells will allow certain limits to be overcome. The mesoscopic size of these materials gives them a number of specific characteristics that allow optical and electrical properties to be engineered. INES has set up an ambitious research program to investigate these new concepts.
The development of a flexible substrate (polymers) technology that is compatible with printing processes will reduce the cost and fragility of cells. As a result, scientists are investigating the possibility of using wet methods to produce organic photovoltaic cells on glass and organic substrates.
Current research into photovoltaic modules at INES focuses on three main objectives:
• Reducing assembly costs for modules per unit of power (€/Wp), which is the most important parameter,
• Increasing output per unit area and per unit weight (Wp/m2, Wp/kg),
• Ensuring maximum life span.
As for many other research programs at INES, partnership contracts have been signed with relevant companies. For example, INES is currently supporting the industrial development of a very innovative technology that will drastically reduce manufacturing costs while considerably facilitating recycling.
Recent years have seen a notable change in the PV market, as the proportion of grid-connected PV systems has out-stripped the proportion of stand-alone systems. Grid connection is a recent trend in which there is still much room for technological and functional improvement.
The main objective is to continue increasing the ratio of kWh produced per kWp installed in order to maximize the amount of energy produced by photovoltaic systems. Improvements can be made in:
• The electrical architecture of systems,
• Power electronics,
• The detection and localization of faults,
• And the addition of system services.
The optimal management of whole systems is a transversal objective.
L2S has expertise in every aspect of solar systems (modules, power electronics, connectors, networks, supervision). Components are characterized individually and globally and then modeled. Starting from the idea that combinations of independently optimal components will not necessarily form optimal systems, the laboratory focuses on producing evaluations of complete systems. Grid-connected systems are still “archaic” in terms of embedded intelligence. Major efforts are being made to find ways of adding new functions that will:
• Increase security for the continuous supply of current,
• Make the architecture dynamic in order to reduce shade effects,
• Improve monitoring to ensure better maintenance and better prediction
• Increase possibilities for building integration.
Another point concerns the addition of storage systems to provide extra flexibility and therefore make it possible to create system services that meet the needs of grid operators. L2S is also developing new control laws and using “load flow” type simulations to measure the impact of storage systems on voltage profiles.
In order to use intermittent energies, particularly photovoltaic systems, in electricity production systems, it is essential to be able to store energy so production can be matched to consumption.
Although this is particularly true for stand-alone systems (not connected to an electricity grid), grid-connected systems will also benefit, as storage facilities would allow intermittent production to be smoothed and energy to be injected into the system at the most appropriate times.
A number of storage technologies are available, generally using chemical methods in the form of batteries (Pb, NiCd, NiMH, Li-ion, NaS, etc.).
The STORE research center characterizes the performance and ageing of different electricity storage systems, most of which are based on electrochemical methods.
STORE is the largest center in Europe for research into renewable energies storage systems and the distributed generation of electricity. Its facilities can also be used for studying other storage applications, for example, batteries for electric vehicles.
As is done for other energy resources, STORE characterizes energy storage units in terms of parameters such as efficiency, immediately available energy and power, and life span. The results of these characterization tests are then used in technical-economic analyses, for example, to determine the service provided in terms of €/kWh supplied.