Cutting up the energy sector
From manufacturing wind turbines to decommissioning nuclear sites, laser technology is seeing rapid adoption within the energy sector. The application areas are wide reaching and range from novel techniques still undergoing feasibility studies, to well-established technologies that continue to improve in order to meet the demands of a sector going through massive technical and ideological changes.
Laser cladding is one such established process. The technique deposits material by feeding a stream of powder into a laser beam as it is scanned across the part. It improves the surface and near-surface properties of the component, or repairs worn or damaged parts. Laser cladding is used across a range of industries and, within the energy sector, there are several applications including steam and gas turbine repairs, shaft repairs, gear component fixes, and water wall cladding and boiler tube cladding.
Heiko Riedelsberger, market development manager Europe at Coherent, commented that there is an increased interest in laser cladding for the energy sector, with the technology substituting conventional arc processes.
Laser cladding’s high throughput is a key benefit. It’s also an automated – and hence more repeatable – procedure than an arc process such as PTA (Plasma Transferred Arc) welding. Less material is also needed as laser cladding has a deposition efficiency of between 85 to 95 per cent.
Other advantages to laser cladding compared with traditional cladding methods are that it puts less heat into the part, which is important as there is less part distortion, and iron dilution within the first material layer is lowered. By reducing iron dilution, the service life of the components is improved, and less layer material and time is required during the process.
Most laser cladding repairs specific components. ‘Each application is completely different,’ said Riedelsberger. ‘The optimisation of the process can be time consuming until the right parameter for the cladding has been found. Each parameter then has to be proven in the field on real parts.’
Andres Gasser, group leader of laser metal deposition at the Fraunhofer Institute for Laser Technology, noted that specific focus is being given to process chains, so not only looking at the component on the cladding machine, but also investigating what happens prior to and after the laser process.
High Power Direct Diode Lasers (HPDDLs) are ideal for large area cladding applications, as they offer high throughput and a very low heat input. The near infrared wavelength used by such lasers is also relatively well absorbed by most metals and their output can easily be shaped into a long line, which is then scanned across the part for rapid processing.
These lasers are also physically small, allowing them to be mounted on a robot arm or gantry, which is ideal for repairing the often large parts and contoured surfaces found in the energy sector.
Additive processes such as laser deposition technology (LDT) – cladding is a variant of LDT – are now also used in the energy sector to build freeform fabrications, repair metal components typically considered non-repairable by conventional techniques, or strategically add features to forgings or castings.
Nick Wald, general manager of LDT specialist company RPM Innovations, said: ‘LDT deposits exhibit excellent material properties. These material properties, combined with the flexibility of the process, significantly lower overall production costs.’
Laser technology is still being investigated for a range of novel application areas within the energy sector.
A recent feasibility study – called the Toolform programme, led by TWI in the UK – assessed laser welding against stringent welding standard requirements for components used in heat exchangers, chemical tanks and pressure vessels used in the oil, gas and chemicals industry, as well as water, food, pharmaceuticals, power generation, and nuclear industries.
These components are exposed to corrosive environments, elevated temperatures or pressures from 2 to 20 bar, so it is vital to obtain reliable welds.
Stainless steel and titanium tubes, with varying bore sizes and wall thicknesses, were joined using an HL4006D lamp-pumped Nd:YAG Trumpf laser, coupled with a Kawasaki JS30 robot to manipulate the beam. The combination of a flexible optical fibre to deliver the light and a robotic control system allowed for angled butt joints and various configurations of fillet welds, as well as for straightforward circumferential butt welds.
Laser welding offers considerable savings in set-up times and efficiency over conventional arc welding for these types of component. Non-standard weld configurations can also be achieved and, assuming an acceptable fit-up can be made prior to joining, laser welding can produce single pass welds in the wall thicknesses evaluated with full penetration at welding travel speeds of up to five metres per minute.
Another feasibility welding study, known as the LaserJacket project, is evaluating laser welding to produce offshore wind turbine support structures. The one-year feasibility study, funded by the Technology Strategy Board, is being carried out by engineering companies TWI and Graham Engineering.
Non-serial wind turbine jackets (or space frames) currently rely on labour-intensive conventional arc processing and account for around two-thirds of the total cost of wind turbine foundations. For such jacket structures to provide a solid business solution, laser welding could bring these costs down through increases in productivity and reduced manufacturing expense. Laser welding techniques suitable for section thicknesses outside the current capabilities of lasers, up to an industry relevant steel thickness of 60mm, are currently being investigated.
Shifting focus to the small-scale, micro gas turbines are used to optimise the combustion of fuels, and the geometry of such devices must be optimised to raise combustion efficiencies and reduce exhaust gas emissions. Metal Additive Manufacturing (AM) technology – or 3D printing as it is more commonly known – has helped to optimise the process.
An EOS M 290 metal AM system was modified by German company Euro-K to aid the design process. The scale of the machine’s interior had to be enlarged to accommodate the 800mm burner.
The result is a new burner that can use gaseous and liquid fuels equally effectively, with an optimised geometry that also allows the use of liquid fuel oils such as those distilled with alcohol, and which are classified as difficult to burn. Another positive effect is that the burner’s innovative design allows the size of the combustion chamber to be reduced by 20 per cent.
Cleaning turbines, along with drilling holes in their blades, represent two other application areas for lasers. Fibre lasers are used extensively in industrial cleaning processes, such as cleaning prior to weld preparation, as well as the simultaneous removal of surface debris, contaminants and corrosion from turbines.
Compared to chemical cleaning, fibre laser cleaning offers superior productivity and reduces the quantities of hazardous waste chemicals. The EU REACH directives progressively seek to reduce the quantities of hazardous chemical waste. Consequently, industry is discovering that high power fibre lasers provide the most eco-friendly alternative – fibre lasers have reached 50 per cent wall-plug efficiency. Mark Thompson, UK director of sales and service at IPG Photonics, which makes fibre lasers for such a process, said: ‘Our high power fibre lasers dramatically reduce the use of chemicals in the cleaning process. Controlling the focused light from a fibre laser is simple to automate. Multiple types of contamination are readily vaporised, leaving a metallic surface prepared for bonding or finishing applications.’
Fibre lasers are also used to drill holes in turbine blades. The blades are running at temperatures higher than the melting point of the material, so they are usually clad with a ceramic-based coating. Drilling into the blades typically involves using a nanometre pulsed laser to remove the barrier coating before another laser drills the holes. Mark Greenwood, chief technical officer at SPI Lasers, said: ‘That represents quite a big business area, because it allows the end user to work at higher temperatures and this improves efficiencies, which is a big part of the ongoing engine development process.’
Drilling throughput improvements have been demonstrated by the use of an active fibre laser. Thompson said: ‘Drill hole quality of a fibre laser has been matched with older laser technologies but at very much slower drill hole rates. Our [IPG] QCW fibre lasers maintain hole quality and precision, while operating at ultra high speed to yield the most economic productivity.’
Past energy behemoths also need the help of the laser industry. More than 20 nuclear sites in the UK will need to be decommissioned by 2030 and this has prompted engineers to find novel techniques to help such sites safely end their working lives.
Fibre laser technology provides significant benefits to the decommissioning process compared with conventional techniques. These advantages include: fast cutting of metallic objects, scabbling concrete to reduce surface contamination, reduction of fumes or secondary waste associated with some abrasive conventional cutting techniques, portability, and process repeatability.
Stan Wilford, senior sales manager at IPG Photonics UK, said: ‘These all point to reducing some of these projected high costs. The high powered laser energy from an IPG fibre laser can be delivered efficiently by means of a flexible optical fibre, in conjunction with specialised robots making it possible to laser cut contaminated metal components and structures remotely, safely and without human intervention.’
The LaserSnake is one contactless solution – a single fibre laser that can be configured in two different ways to carry out the two different processes of concrete scabbling and tube cutting within a nuclear decommissioning environment. Developing a tool to cover both application areas is difficult, as laser cutting demands a very high power density in the beam, whereas laser scabbling requires a relatively modest power density.
An industrial fibre laser is suitable for both roles. It is also robust, compact and appropriate for the remote work required in a nuclear decommissioning environment. Wilford said: ‘A specially selected optical fibre, in conjunction with an IPG remotely controlled optical beam switch, can deliver the laser’s energy to a hazardous area for a process that is all remotely controlled.’
The fibre lasers are located and operated remotely from the hazardous environment, while the cutting head makes no mechanical contact with the decommissioned materials, making this technology a safe alternative to conventional techniques.
The LaserSnake uses an articulated robot arm with IPG’s YLS 5000 fibre laser to cut through metal sheets and pipework. Stainless steel tubes from 25mm diameter to 170mm diameter, with a range of wall thicknesses from 1.5mm to 11mm, were cut using single pass, two pass and multiple pass techniques.
In the laser scabbling process, the laser beam hits the concrete’s surface and its energy is absorbed. The concrete matrix and the concrete aggregate are heated and expansion of residual water vapour causes the concrete to break up in a highly energetic fashion, leaving a rough scabbled surface, consisting of matrix and aggregate. For concrete with a limestone aggregate, the 5kW laser can remove a square metre of surface to a minimum depth of 10mm in less than two hours.
The LaserSnake project is a collaborative one, led by research company OC Robotics and involving the expertise of TWI and IPG. The project is now working on its second iteration, LaserSnake 2.
As laser manufacturers aim to meet the future needs of the energy sector, there is a key requirement to improve battery technology for renewable forms of energy. Greenwood at SPI Lasers explained: ‘You have a lot of green technology but you cannot access it when you may want it. The wind stops blowing, or at night there’s no solar energy, so the biggest challenge for the energy industry is finding a way to store this energy efficiently.’
Greenwood added: ‘The next step will be to put battery storage into houses so you can store the green energy locally to be used when you need it. I believe there will be a lot of work on distributed storage to support the energy industry. It’s the big unsolved issue of green energy.’
One focus within the battery arena is to weld dissimilar metals successfully. Greenwood said: ‘It may be a more expensive process compared to conventional methods, but it gives you the ability to control the amount of heat, to create welds often without a filler or additional material, and it’s a contactless, repeatable and controlled process.’
Lithium ion batteries are made of thin foils that are cut with lasers and then the parts are welded together. This poses quite a challenge in terms of controllability as the end user demands a strong weld – but the foil must remain intact and not be cut during the process.
Many forms of energy production, both renewable and non-renewable, employ laser technology for manufacturing components like turbines or drills. The solar industry uses lasers for numerous production steps, including solar cell edge isolation – typically achieved by scribing a groove around the perimeter of the solar cell – and marking, cutting and scribing of wafers. The benefits of the laser and its uses are still being harnessed by the energy industry as it continues to develop the technology required to improve its vast range of application areas.
Laser technology is being investigated as an alternative to conventional methods in the solar cell sector to improve cost efficiencies and process simplification for the production of photovoltaic (PV) products. Christian Hördemann, development engineer at the department of micro and nanostructuring at the Fraunhofer Institute for Laser Technology, said: ‘After breakthroughs in the PV sector in the recent years, solar cells have gained importance as a sustainable and regenerative energy source.’
One widely adopted process in solar cell manufacture is laser edge isolation. Edge isolation takes place during the doping/diffusion step of the silicon cell manufacturing process, in which a shallow layer of silicon is infused with negatively doped ions. This doped region surrounds the entire wafer and causes electrical shunting between the front and back electrical contacts without an isolation scribe.
Laser edge isolation is typically achieved by scribing a groove around the perimeter of the solar cell, as close to the edge of the wafer as possible. The groove depth must extend some distance beyond the ion diffusion to give the best result. Other applications include the marking, cutting and scribing of wafers and drilling for electrical contacts.
The solar cell market is divided into two technologies: silicon solar cells and organic thin film solar cells. While silicon solar cells are an established technology, thin film cells are a relatively novel concept and laser technology continues to aid the design and manufacturing process to make thin film cells a production-ready solution.
There are many advantages to organic thin film solar cells. They can be produced in continuous roll-to-roll processes, which is a simplified manufacturing process compared with silicon cell production. Due to the low weight of the substrate, organic thin film solar cells can also be implemented in large areas and within mobile applications. Hördemann added: ‘Furthermore, roll-to-roll processes generate high throughput, allowing the manufacturing of organic solar cells to be highly cost efficient.’