Eco-Solar envisions an integrated value chain to manufacture and implement solar panels in the most ecologic way, taking into account reuse of materials while manufacturing and repurposing solar panel components at end of life stage.
Moreover, the project will demonstrate that during the lifetime of a solar electricity producing PV field, individual panels can be monitored, so that defaulting panels can easily be identified and repaired or replaced.
90% of all solar modules are made as follows
Silicon feedstock is being crystallised, either via Directional Solidification (DS)- and Czochralski (Cz)-crystallisation routes. The resulting “ingots” are carved into very thin discs called “wafers”. These wafers are the basic principle for solar cell processing that will turn the slice of silicon into a device capable of converting the energy of sunlight into electricity.
When multiple solar cells, usually 6 x 10 cells, are electrically connected and packaged together, they form a solar panel, also called solar module.
Envisioned environmental reductions within the solar value chain
1. Recovery & Re-use during Si feedstock crystallisation
When starting at the beginning of the value chain, the crystallisation of feedstock, there are three major opportunities to significantly reduce the carbon footprint: reuse of argon purge gas, reusable crucibles and recycling silicon-kerf-loss that will be recovered from the next step, wafering.
In order to understand the impact of the changes, and to achieve full understanding of the effect of the intended changes on the quality of the material, SINTEF will compare the research results with baseline ingots for the Directional Solidification (DS)- and Czochralski (Cz)-crystallisation routes. Both routes are important, as together they hold 90% of the market volume for PV, and their different processing routes give different challenges in crystallisation and further down the value chain.
1a. Reuse of Argon gas
Pure argon gas that is used to remove contaminants during ingot crystallization is currently being vented into the air. Though perhaps argon is abundantly available in the earth’s atmosphere (approximately 1% of mass), there are still (ecological and financial) costs involved in the production of pure gas. For solar applications, approximately 5% of the wafer costs are related to argon gas. Though in other industries, argon gas recycling has become state of the art, in PV-production this is not yet the case. The recycling processes from the semi-conductor industry are too refined and hence too costly, but they could be modified for PV-requirements.
Within Eco-Solar, SINTEF will evaluate an existing argon gas recycling system, for its potential for using it within both Cz and DS crystallisation processes. SINTEF will determine crucial parameters from the baseline process, and will make modifications to make the system applicable and cost-effective for PV.
Once a working prototype has been established at SINTEF, Norsun will install it, and validate the results in industrial setting. Having a cost-effective system for argon recycling could bring a cost-benefit to Norsun compared to its global competitors.
1b. Reusable crucibles
Crucibles are the main containers for the (molten) silicon feedstock. As silicon at high temperature reacts easily with almost everything, there are only a few crucible materials that will meet purity requirements for manufactures. One of the main drawbacks of using silica, as is current standard practice, is the (ecologic and financial) cost due to single use. Silica crucibles contribute up to ~30% of the conversion cost from Si-feedstock to the as-grown ingot.
STEULER has developed a concept for reusable silicon nitride crucibles, for both crystallization processes, DS and Cz. In Eco-Solar the impact of different crucible manufacturing procedures and crystallization process parameters to meet the requirements for Cz and DS ingot production will be determined, with the main aim to be able to reuse the crucibles at least 10 times.
2. Recovery & re-use of Si-kerf-loss
After crystallisation, side-, top- and bottom-parts are removed before the ingot is cut into blocks and then wafered. A considerable amount of silicon is lost in form of fine powders (kerf-loss) during the mechanical processing of silicon ingots to obtain wafers: up to 50% of the material is lost in air and water used to rinse the wafers. The ability to recycle the kerf-loss for solar ingot production will have beneficial effect in terms of savings of precious poly-silicon consumption and of waste reduction. Therefore, Garbo has patented and implemented a silicon recycling process which removes contaminations and brings silicon to “six nines” (or 6N), i.e. 99,9999% purity. Purified silicon is then compacted through another patented technology in order to obtain a mechanically stable, pelletized material, which can be used in the standard production of solar ingots and cells. Within Eco-Solar, the consortium will look at 3 factors that will give both environmental and monetary gain: reducing chemicals consumption in cleaning step, improving compact density of powders and increasing kerf-loss saturation level in sawing coolant.
2a. Reducing chemicals consumption in cleaning step
Garbo will carry out cleaning tests in the project, where the recovered kerf-loss will be exposed to different mineral acids recipes. Specifically, this task will involve the minimization of chemicals consumption in the cleaning step while achieving purity levels suitable for solar.
2b. Improving compact density of powders
The cleaned silicon powder will undergo a compaction in order to maximize the density of compacted Si powder using the lowest possible binder amount. This factor is important to optimise the mass/ volume ratio, which allows the crucible volume to be better utilised during crystallisation, hence optimising resource efficiency. Garbo will therefore carry out experiments to optimise the binder-assisted compaction of the cleaned powders that improves density and mechanical stiffness of the compacts.
SINTEF will carry out crystallization experiments of the recycled compacted silicon powder in order to assess the melting properties of the compacted silicon powder.
2c. Increasing kerf saturation level in coolant
In order to extend the coolant’s lifespan and decrease coolant’s recycling costs, Norsun will carry out sawing tests with increased silicon-kerf-loss saturation level. Norsun will study the effect on the quality of the obtained wafers in terms of saw marks, bow, total thickness variation, powder particle size, wafer yield etc. as well as the cleaning efficiency. Further, diamond wire consumption as a function of silicon-kerf-loss saturation level in sawingwill be monitored. Norsun will provide the coolant with increased saturation level to Garbo to study if (how) the saturation level in the coolant affects the coolant recycling process and the recycled coolant quality, even if no variations are expected in this respect.
3. Remanufacturing, resource efficiency and reuse in solar cell processing
Wafers are being processed into solar cells, causing waste like etching chemicals, broken wafers and broken solar cells. By reducing and avoiding certain chemicals or metals (e.g. HF, Pb, Ag, Al) and using less pure or recycled chemicals, waste can be reduced and the ecologic footprint of solar cells and panels can be improved. Moreover, as it is extremely costly to undo the processing steps and recover the chemicals and metals as separate elements after solar cell processing, solar cells represent a very valuable half-product. Thus, repair of solar cells can provide significant environmental benefit, while reducing the outcast of solar cells can simultaneously provide cost benefit.
3a. Solar cell repair
In order to capitalise on the value a solar cell represents, repair of defect solar cells within the solar cell process can represent a significant financial and ecological impact. AIMEN coordinated the EU-funded project REPTILE, in which ISC and INGESEA participated as beneficiaries. The project resulted in a fully functional prototype system, able to automatically select and cut or isolate non-defective areas in defective cells and wafers, called the Cell-Doctor.
Within the Eco-Solar project, ISC will evaluate the rejected solar cells with an automated system for defects recognition. AIMEN and INGESEA will further develop the accuracy of the Cell-Doctor prototype, aiming to avoid 50% scrapped cells.
3b. Reducing silver consumption
Silver is used in most solar cells (>98%) currently produced, providing the metal contact that “collects” and “drains” the current from the solar cell. It is of great importance to establish good electrical contact between the metal grid and the emitter surface with a low contact resistance, while having narrow, well conducting, fingers to minimise shadowing losses to produce higher efficiency solar cells. Todays silver consumption for PV is 3.5 – 15% of the total silver market and is expected to grow further assuming a market share of crystalline Si PV of approx. 70% in 2030 .
ISC will experiment with different solar cell architectures, to minimise the use of Ag. In addition, an innovative interconnection scheme enabled by Apollon’s module design, will avoid soldering of a contact tape onto the rear side Ag pads and the front side busbars. Only the small contact fingers on the front side are needed. The full rear contact area is realized by aluminum.
In the Eco-Solar project, experiments will be carried out that will test: different solar cell architectures, the novel interconnection scheme from Apollon and metallisation pastes that contain less Ag than current standard, aiming to save at least 66% of Ag.
3c. Reducing chemicals consumption through resource efficiency and chemicals recovery
In crystalline solar cell manufacturing, several wet chemical etching and cleaning steps are implemented, which have negative impact on the environmental footprint. ISC, together with SoliTek will develop an alkaline method for saw damage removal, texturisation and cleaning, that will replace the current state of art concentrated HF:HNO3. Moreover, ISC and SoliTek will investigate advanced processes for emitter formation that will enable higher throughput, resource efficiency and avoid the needs for chemical phosphorous glass layer removal (PSG-removal).
The separation of front and rear of the solar cells is normally carried out with wet chemistry. AIMEN will further develop advanced laser treatment, in order to minimize the cut-off area at the wafer edges to a negligible fraction of the whole wafer area. This will be possible due to the small volume of laser-ablated silicon on the real wafer edges (and not on the front surface), with high speed scanning strategies.
3d. Recycling water
Tap water is being de-ionised by the solar cell producers (and by solar cell research institutes), resulting in very clean water. This de-ionised water is mainly used for several cleaning steps in the solar cell process: after each chemical processing step, the wafers are flushed with this ultra-clean water. The water that goes into the drain, is mainly of higher quality than the water contracted from the tap. As an example, solar cell processing at SoliTek, with a production capacity of 80MW, consumes about 54.000m3 de-ionised water/year.
As protection of water resources, of fresh and salt-water ecosystems and of the water we drink and bathe in is one of the cornerstones of environmental protection in Europe. ISC will look into industrial viability of recycling systems for the waste water.
4. Module design for remanufacturing
After completion of the cells, they are electrically series connected and assembled into a solar module. Moreover a junction box is attached that will ensure suitable power output. Within current solar modules, the biggest eco-challenge is that they cannot be “re-opened” and the only potential for recycling is through destructive processes. Current PV modules also contain a vast amount of organic materials (EVA for encapsulation, PVF in backsheets)
As Apollon’s NICE-modules are already designed for disassembly, in Eco-Solar Apollon will focus on the following:
- Further reduction of the module BOM (Bill of materials) focussing on a frameless version of the PV module
- Specifications for automated module disassembly equipment including diagnosis tools for quality screening of module components.
- Integration of electronics in the junction box, that can monitor the performance of the individual module and provide warning if performance drops drastically.
- Investigating the ageing and degradation aspects of solar modules during installation, and assess the technological viability to capture and recycle components at end of life, while considering environmental and economical aspects as well.
Moreover, Apollon will to demonstrate that module assembly of the Eco-Solar cells will result in comparable module output as with current non-eco-friendly state of the art module technology.
Assessment and outreach
bifa will collect data from all workpackages in order to assess the environmental impact of the intended innovations. Moreover, it will support partners with identifying waste streams that are costly and hard to recycle, and find opportunities to repurpose those waste products. Boukje.com will disseminate the results to the scientific and public domain, and support the partners with the exploitation and replication potential of the results, with this public website as main platform.