Use of Steel in the Generation of Solar and Wind Power
Use of Steel in the Generation of Solar and Wind Power
At present energy transition is taking place around the world. Renewable energy is at the centre of the transition for a less carbon-intensive society. Strategic moves and heavy investments are being made in the field of renewal energy. Renewable energies include solar, wind, geothermal, hydro, and others. Out of all types of renewable energies, the solar energy and wind energy have a very high percentage of investment with the solar power installations occupying the leading position followed by the wind power installations.
This has put steel in the centre stage of this transition since steel is needed by each of these technologies for the renewable power. Steel plays an important role in all renewables, including and especially solar and wind. Steel is important in the conversion of solar energy into electricity as well as hot water. It serves as a base for solar thermal panels, heat exchanges, tanks, and pumps. The future of steel in the energy transition is exciting. Steel occupies an excellent position to provide safe, sustainable solution for the future of energy.
Without steel, none of the renewable energy sources are possible. Every renewable energy structure, whether a wind turbine or a solar panel needs steel. Each new mega watt (MW) of solar power needs between 35 tons to 45 tons of steel, and each new MW of wind power needs 120 tons to 180 tons of steel. Transmission and distribution lines also need steel, and probably more of it, as installations moves further offshore. The average high-voltage transmission tower includes around 18 tons to 27 tons of steel. Transmission wire contains steel. An ACSR (aluminum conductor steel reinforced) wire having a weight of around 2.4 kilograms per metre has in it around 0.4 kilograms of steel per metre.
Solar power plants
Solar energy is a sustainable green energy which is never-ending available and does not need any fuel to produce it. Solar panels (SPs) can be of various cross-sections (e.g., square, and rectangle etc.) and sizes but their main purpose is to convert the sun light into electricity. Normally, solar power systems can be separated into three used groups like (i) concentrating solar power, (ii) solar-thermal absorbers and (iii) photovoltaic solar power.
Solar power plants use three technologies namely (i) solar Photovoltaic (PV), (ii) concentrated solar power (CSP), and (iii) concentrator Photovoltaics (CPV). All of these technologies use steel in the structure on which the PV modules or mirrors are attached. Solar PV panels are mounted on a fixed or moving structure which allows the panel to be optimally oriented to the sun throughout the day. The moving structure is gaining ground on the fixed with tracker technology. CSP panels are with lenses which focus the sun’s ray on a small cell with a much higher energy generation capacity. CPV converts sunlight to electricity by PV cells made of semiconductor materials. The material is normally salt, which becomes liquid and generates steam to create electricity.
The proper mounting of the solar panels on the support structures determines the performance of the solar power plant. Two different design approaches are used for the support structures of the solar panels. These are fixed support structure design and adjustable support structure design. A wind load operates on the solar panels because of the wind tunneling effect. This wind load as well as the weight of the solar panels can result into the structural deformation and misalignment of solar radiation. Further the support structure is also to be designed to resist the seismic load which is normally site specific. The stability of the structure is an important aspect. Hence, solar power plants need well designed reliable structures to support the solar panels.
Fig 1 Typical support structure for solar PV panels
Steel frames made of structural steel are normally used for supporting the solar PV panels at certain height above the ground. The support structure made of structural steel can sustain a wind load with velocity of 55 metres per second. Durable steel is a foundation for sustainable solar energy. Resistance against corrosion is crucial for maintaining a long-lasting solar energy generating system. Once installed, the solar panel and its frame structures are exposed to external forces which can reduce electric power generation.
Steel for producing the structural frames is required to have certain properties. The steel material needs to have sufficient high strength which can resist the pressure of air at the plant site. Structural steel normally has sufficient strength and has a high resistance to breakage. It is quite ductile, even when cold. It has high tensile strength and impact strength. Shear strength of the structure made from structural steel is an important factor because of the possibility of structure breakage due to the shear stress. Structural steel also has the shear strength to resist the breakage of the structure. Further, structural steel has the necessary hardness needed for the construction of structure of the solar power plant. Structural steel can also be machined and shaped easily due to its inherent flexibility. It can be hardened with carburizing, making it the ideal material for producing support structure of the solar power plant. Structural steel is also weldable and can be welded by using any of the different welding processes. Another advantage of structural steel is that it is easily available.
Support structure of the solar power plant is required to have rigidity for resisting the deformation when there is application of external force. The speed of the wind at the solar plant site applies large amount of force on the panel and try to deflect them or separate them from support structure. Hence, such material for the support structure is needed which is highly stiff. The stiffness of structural steel makes it suitable for the support structure. The frames and leg assemblies of the array of support structures are made of structural steel which is hot dip galvanized. The steel sections used for the structural frame are normally angles, channels, ‘I’ or ‘H’ beams, tubes or pipes, or any other sections conforming to the national standards for steel structure to meet the design criteria.
To withstand exposure to wind, water, and salt, structural steel is required to be painted or to have a coating layer which increases considerably its resistance to corrosion. Protection from corrosion is one of the important criteria for the support structures of the solar power plant. Corrosion destroys the material strength and makes the support structures weak leading to their failure. To protect the steel structures from corrosion, galvanized steel (zinc coated) with chromate coating is normally used. Recently magnesium and aluminum coating and zinc, magnesium, and aluminum material coating material have been developed for the coating of the steel. These coating materials provide 5 times to ten times higher resistance against corrosion.
High strength steel and stainless steel can also be used for the fabrication of steel support structure. High strength low alloy (HSLA) steel is increasingly being used due to its characteristics such as light weight, high strength-to-weight ratio, durability, good formability, and recyclability properties. Solar PV panels are mounted at certain height above the ground on support structure. Solar panels are arranged in rows. The steel support structure has five basic bearing members named as (i) rail for solar panel mounting, (ii) beam, (iii) column, (iv) purlin, and (v) brace. Steel support structure is erected on the reinforced concrete foundation. Reinforcement steel bars / wire are used in the concrete foundations.
Fig 2 Support structure and solar PV panel
The solar PV panel needs a robust frame to withstand the difficult conditions at the plant site. Although stainless steel has a higher density than other metals such as aluminum alloys, it has got higher mechanical strength. Hence, wall thickness of the stainless steel frame can be reduced to a point where they are of a similar weight to light metal options. The stainless steel frames can be quite thin because of the unique mechanical properties of stainless steel (Fig 2). The stainless steel can also resist high wind loads.
Stainless steel is selected for use in solar panels primarily because of its superior corrosion resistance. Stainless steel is corrosion resistance through and through. Even if the material is damaged, its intrinsic self healing capability ensures that the surface does not discolour or corrode. This property of stainless steel is called passivation. It is this reason why stainless steel does not need any coating or some other form of surface protection to remain bright and shiny.
Stainless steel makes a good choice for applications in solar plants. The CSP plants are now more widely used. Due to this, stainless steel is proving to be the perfect material to accompany these high-tech plants. The natural properties of stainless steel as given below make it perfect for a number of applications in and around the CSP plant.
Corrosion resistant – Stainless steel does not rust. Hence it is the ideal choice for the tough environments where these solar power plants are frequently based. Sand, moisture, and the corrosion are normally associated with the solar plant site. This makes stainless steel a great choice for fixings for the heliostats outside. Furthermore, its resistance to corrosion from the molten salt makes it ideal for components and storage tanks within the system, too.
Able to withstand extremes of temperature – The material for storage containers needs to withstand temperatures in excess of 500 deg C. On the other side, the anchoring bolts for the external rotating mirror panels need to cope with sub-zero temperatures in the desert overnight. Stainless steel is the perfect solution for this, as it does not warp, break or melt in extreme temperatures.
Naturally hygienic – Unlike some other metals, stainless steel is resistant to abrasion and to developing cracks over its lifespan. Cracks, chips, and dents can harbour bacteria, which can start to pose a health hazard. In solar power plants, the molten salt mixture does not become contaminated thanks to the robust properties of stainless steel.
Long lifespan – Stainless steel is designed to last a lifetime. Majority of the CSP plants are built to last around thirty years or so. Stainless steel almost certainly outlives the power plant itself, making it a great choice for these types of applications.
Because of its unique properties, stainless steel is really the only choice for several applications in and around the solar power plant. There are several examples in practice using stainless steel in solar plants.
At the Crescent Dunes Solar Energy Plant in Nevada over 60 tons of stainless steel went into the anchoring bolts for the heliostats. In the same plant, 650 tons of high grade stainless steel has been used in the hot nitrate storage tank.
The Gemasolar CSP plant in Fuentes de Andalucía, Spain, used more than 160 tons of 374H stainless steel in their molten salt storage tanks. This was the ideal combination of heat and corrosion resistance for this application. Stainless structural laser welded ‘I’ beams similar to IPE240 and IPE270 sizes in 347H grade are used for salt tanks. These beams are fabricated from hot rolled plates. Throughout CSP plants, stainless steel is frequently specified by the designers. It is used for components such as heat exchangers, water handling systems, flanges, and much more.
Wind power plants
The beauty of wind energy is that it does not cause any sort of pollution to air, water, or land. Nor does wind energy generate harmful greenhouse gases. Wind turbines do not emit carbon di-oxides. All it takes is a couple months of operation for wind turbines to recoup the energy needed for their construction and operation. The wind turbines would not be nearly as tall, formidable and efficient if steel was not available for their construction.
Steel is a major contributor to wind energy. The application of steel in the majority of the key components of wind turbines makes it possible for the wind energy industry to meet the technical requirements of the turbines and climate change demands at the same time. Steel is the foundation for ever-taller, stronger, and more efficient wind turbines which are capable of generating an ever-increasing quantity of energy as time progresses.
Wind energy can be grouped into three key types of the plants namely (i) on-shore wind power plant, (ii) off-shore wind power plant, and (iii) new generation wind power plant. On-shore wind power plants are relatively small and produce between 2 MW and 3 MW of power per turbine. A variety of steels is used for the structural tower itself in the house of the turbine, the turbine blades, and electrical steel. Off-shore wind power plants are much larger, with recent turbines producing 5 MW to 8 MW of energy. Off-shore wind power plant consumes more steel than on-shore power plant. Off-shore power plant has two advantages namely (i) the ability to go much higher with no obstructions, and (ii) the free flow of wind. Off-shore power plant needs a combination of steels to support the huge foundations which are anchored to the sea bed. Steel is also found in the structural tower itself and the blades, which are much thicker and longer. New generation wind power plants go the distance, literally. These power plants are to be built even further from shore, needing a floating structure. Each turbine is to produce 7 MW to 12 MW of energy.
Wind turbines come in many sizes and configurations and are built from wide range of materials. In simple terms, a wind turbine consists of (i) a rotor which has wing shaped blades attached to a hub, (ii) a nacelle which houses a drive train consisting of a gearbox, connecting shafts, support bearings, the generator, and other machinery, (iii) a tower, and (iv) ground-mounted electrical equipment. The wing shaped blades on the rotor actually harvest the energy in the wind stream. The rotor converts the kinetic energy in the wind to rotational energy transmitted through the drive train to the generator. The generator is made up of 65 % steel and 35 % copper. Generated electricity can be connected directly to the load or fed to the utility grid. The material for wind turbine is dominated by steel. Fig 3 shows the nomenclature of a wind turbine.
Fig 3 Nomenclature of a wind turbine and lattice tower
A transformer, normally on the ground, converts the electricity from the turbine to the higher voltage needed by the electricity grid. Different bearings are used in the wind turbine. All have to withstand the varying forces and loads generated by the wind. Screws and studs are needed to hold the main components in place and are to be designed for extreme loads. All of these components depend on steel.
The main components of the turbine are the tower, the nacelle, and the rotor. The majority of wind turbines consist of three blades mounted to a tower made from tubular steel. There are less common varieties with two blades, or with concrete or steel lattice towers (Fig 3). While the blades are normally made of other materials, such as carbon fiber or alloys, steel holds the turning blades in place, utilizing a cast iron or forged steel rotor hub. At the top of the tower are the rotor and the nacelle. Nacelles can have a width of up to 3,300 mm and a thickness of up to 200 mm. The material used for nacelles is structural grade steel sheet, predominantly. Since a nacelle can weigh as much as 300 tons, the strength of steel makes steel the perfect material for the frame, housing, and machinery of the nacelle. The nacelle contains some of the highest value steel, including special steel (electrical sheet steel) tailored to have the specific magnetic properties which make wind energy feasible.
Behind the blades, a low speed shaft transfers the rotational force of the rotor to the gear box. Here, the gears made using precision tools and hardened steel components are operated, and increase the low rotational speed of the rotor shaft to the high speed needed to power the generator. Next, the mechanical energy captured by the blades is converted into electric energy, which is then directed to the transformer and converted to the higher voltage needed by the electricity grid. Fig 4 shows a wind mill and its parts.
Fig 4 Wind mill and its parts
Steel plays a central role in the design of wind power plants. It is a predominant material which composes majority of the structure of a wind turbine such as the tower and the generator, excluding only the blades. The performance of electrical steel sheets and bearing steel used in the internal structure has a direct relation to the turbine efficiency. The wind turbine (machinery used to produce wind energy) has several steel parts. Majority of the wind turbines are made of steel, and for an average wind turbine, 140 tons of steel is used. From its all important foundation down to its screws and studs, every part of a wind turbine depends on iron and steel. In fact, steel, on average, represents 80 % of all the materials used in the construction of the tower, the nacelle, rotor blades and its supporting facilities. The majority of wind turbines consist of three blades mounted to a tower made from tubular steel. There are less common varieties with two blades, or with concrete or steel lattice towers.
According to a report from the National Renewable Energy Laboratory, depending on make and , wind turbines are predominantly made of steel (66 % to 79 %) of total turbine mass, fiberglass, resin or plastic (11 % to 16 %), iron or cast iron (5 % to 17 %), copper (1 %), and aluminum (0 % to 2%).
Steel is so strong that it can hold turbine blades in place when they rotate at fast rates of speed. Steel is also the robust material needed for machinery of the turbine and the nacelle frame. A nacelle can weigh upwards of 300 tons. It needs electrical steel for risk-free operation and sufficient durability. Every large tubular wind turbine makes use of steel in its towers. Each section of these towers makes use of flanges at the ends. Steel provides the strength necessary to serve as a formidable base, supporting the incredibly heavy weight of the turbine above. Steel is flexible to the point that it permits the tubular steel tower conical shape without flaw.
The majority of steel is used to make the tower which serves as the foundation on which the blades turn to generate energy. There are several types of turbine towers, such as steel-concrete hybrid towers, steel truss towers and steel lattice towers, but around 90 % of all wind turbine towers are made of tubular steel. To construct one of these, fan-shaped plate segments are cut from rectangular parent steel plates and are then roll-formed and welded into cone sections.
Most of the steel in a wind turbine is the tower. Around 90 % of all wind turbine towers are tubular steel towers. They are called tapered tubular towers since they gradually narrow towards the top. A section’s thickness can vary from 8 mm at the top to 65 mm at the base, depending on loads and steel grades used. Off-shore wind turbine installations normally use thicker or stronger plates.
Steel is also relied upon for the construction of lattice turbine towers (Fig 3), hybrid towers, and bolted towers. Lattice towers are built with welded steel profiles which allow the wind turbine to function with minimal materials. Steel hybrid towers are optimal for especially tall turbines. Such steel-concrete hybrid towers are likely the wave of the future when it comes to wind power. Steel is used in the upper portions while concrete is used towards the base.
Steel lattice tower solutions were popular in the past and they have experienced a renaissance in recent years. Using mainly standardized steel sections they compare well with other tower concepts when looking at life cycle cost. At present, lattice tower wind turbines have almost been replaced by the tubular tower. They are ecologically attractive, being the most cost effective solution to reach the greatest heights. Especially for on-shore applications, reaching higher heights allows increased wind speeds to be utilized increasing efficiency and turbine power. Additionally, wind shear decreases with height, thus fatigue stresses are reduced as well. The design of lattice tower provides a reduced frontal area, optical transparency, and reduced weight in combination with high bending stiffness. The low cost structure, reduced foundations combined with a corrosion protection through galvanizing makes the lattice tower a good investment over the long-term. Lattice towers are constructed of pre-assembled steel sections which are hot dip galvanized for corrosion protection and bolted together on site. The tower is then lifted by a crane.
For speed and cost efficiency, steel tubular towers are transported to site as complete tubes. This limits the maximum tower diameter to around 4.3 metres. For off-shore developments, the tower can be lifted onto a barge and shipped out whole. Taller towers are segmented for transport. Higher steel grades can be applied to achieve lighter and taller towers. For example, by upgrading the steel of a wind tower structure from structural steel grade to HSLA grade, a weight saving of 30 % can be achieved. Even with a cost increase of 20 % to 25 % per ton for the higher strength steel, the balance is positive since 30 % less material is needed. More savings is achieved because of lower transport and construction costs.
Sometimes, steel-concrete hybrid towers are used to overcome transport restrictions associated with the taller towers. Concrete sections are constructed and combined with steel tubes on site. However, on-site concrete solutions are heavily dependent on good weather, and need a lot of skilled labour and extended construction times. Some turbine manufacturers have installed steel-concrete hybrid prototypes using pre-cast concrete. In the forest areas, steel truss (lattice) towers are used to lift turbines above the tree-line without disturbing vegetation. The lattice towers are proving a cost efficient solution for very tall towers. The world’s tallest wind tower is a steel lattice tower with a hub height of 160 metres.
Wind turbine needs specific and reliable foundation solutions adapted to high loads and dynamics but also reducing the total environmental footprint of the foundation. The tower and the foundation, which connects the turbine to the ground or sea bed, have to be tailored to carry these heavier blades and the bigger rotor which these turbines necessitate. Also, the non-corrosive properties of steel maximize the life time of these wind turbines and minimize the maintenance costs.
The structure of a wind turbine weighs hundreds of tons, its mast is made of heavy plate steel and its foundation is reinforced with reinforcement bars having diameters of 20 mm to 32 mm. A single foundation can use in the range of 60 tons to 130 tons of reinforcement bars. The alloy of the steel makes the windmill foundation durable and stress resistant.
The useful life of wind turbines is normally in the range of 20 years to 30 years. As the turbines age, replacements are needed. The bearings of the turbine are to match the life of the turbine. Hence durability of various bearings of the wind turbine is very important for stable support of turbines having several hundred tons. Durability is maximized by increasing the content of alloying elements (silicon, manganese, and chromium) compared to ordinary bearing steel, enabling long term high resistance to static and dynamic loads as well as to corrosion and damages from external forces.
In case of off-shore wind power plant, the underwater structure is to be installed to withstand strong winds and ocean waves. To tolerate inconsistent weather conditions for over 20 years, the underwater structure needs steel which is highly corrosion resistant. Amid the growth of turbine size and deepening sea levels, highly durable steel is used to increase the strength of the underwater structure while decreasing the weight, in turn, improving the logistics and installation efficiency.
In short, steel is the backbone of the renewal energy and without the steel. The energy transition which is taking place around the world for a less carbon-intensive society is not feasible.