Using and storing solar energy

Energy crisis, skyrocketing electricity prices and the desire to make a personal contribution to the energy transition: More and more people want to use free and almost inexhaustible solar energy for their own needs. Alongside solar thermal energy, photovoltaics (PV) are gaining strongly in importance. PV systems have been particularly attractive for homeowners since the beginning of 2023: the elimination of sales tax on acquisition and installation means that a real boom can be expected for this type of system. But industry and tissue are also increasingly relying on the power of the sun to reduce energy costs. The high expectations in terms of efficiency and cost-effectiveness of the systems are also reflected in the requirements placed on the components used. Thanks to its specific properties, stainless steel is therefore one of the most sought-after materials for the components of a solar system, despite the higher material price.

In Germany, the share of all renewable energies in net electricity generation was 48.3 percent in 2022 (source: Federal Network Agency). The yield of wind energy and photovoltaics increased particularly significantly. Photovoltaics even recorded the highest increase since 2013, rising by 19 percent. PV systems fed 55.3 terawatt hours (TWh) into the public grid, accounting for 11.4 percent of the electricity generated from renewables in 2022. The energy crisis triggered by the war in Ukraine and accompanying electricity price increases were additional drivers for residential, industrial and commercial customers to consider purchasing a solar system.

Heat generation with solar thermal energy

For a long time, solar thermal systems led the field when it came to heat recovery for water heating and backup heating in private housing. Consisting of a collector system of flat-plate collectors or evacuated tubes and a water storage tank, they convert the sun's rays into heat in a way that is both climate-friendly and cost-effective. The sun's rays heat a heat transfer fluid circulating between the collector and the storage tank to up to 200 degrees Celsius. This heat is transferred to the storage tank by a corrugated tube heat exchanger made of stainless steel grade 1.4404, where it heats fresh water in a continuous flow process.

With a combi storage tank model, surplus heat energy can be used to support the heating system. In sunny countries, the heat collected by collector systems heats large-volume stainless steel water storage tanks. As an alternative to blackened collector plates, pillow-type double-walled stainless steel collectors are available which, with a wall thickness of just 0.6 millimeters, combine low weight with optimum heat transfer. Stainless steels of grades 1.4401 and 1.4521 are formed into thin-walled tubes and sheets with complex geometries for the construction and fastening of the tubes and connectors used for this purpose. Stainless steel is also a good choice for the various hot water tanks thanks to its high ductility, corrosion resistance and maximum hygienic properties: For example, it not only meets the high standard specifications for pressure vessels but also permanently prevents the adhesion of bacteria.

Electricity and more with photovoltaics

PV systems are increasingly taking over on German roofs: Lower procurement costs and increasingly powerful solar modules are opening up a wide range of usage options.

They convert sunlight into electricity that not only heats or cools the house, but can also be used to heat water via a heating element, air-to-water heat pump or instantaneous water heater. Hybrid PV systems even produce electricity and hot water simultaneously. However, the latter solution is only economically interesting for households with a very high demand for hot water in summer - for example, to fill their own pool. With the elimination of VAT on new PV systems, components such as modules, inverters and battery storage as well as for the expansion of existing systems, the state has created an additional argument for homeowners to invest in PV since January 1, 2023.

Careful planning a must

The size, design and type of a PV system depend on local conditions, requirements and power demand. The range of customized solutions for roofs, facades, balconies, patio covers and carports is correspondingly large. Special solutions for mobile homes or boats round off the system variety. Each PV system is individually designed to match the size, shape, covering and load-bearing capacity of the roof. For a pitched roof, for example, the orientation of the system is crucial and an inclination of 30 degrees is considered optimal. For flat roofs, a substructure is used that allows any inclination and orientation. Different types of PV modules with thick-film or thin-film cells leave nothing to be desired in terms of performance, robustness, weight, homogeneity of appearance or color. For rigid thick-film cells, millimeter-thin solar cells are alternatively inserted between two films, between glass and glass film, or even between two sheets of glass. Thin-film cells, on the other hand, consist of amorphous silicon that is vapor-deposited onto a carrier made of glass or stainless steel. This type of production allows flexible, curved, folded or even transparent solar modules that can be integrated particularly inconspicuously into the existing architecture of a building.

These solar cells, which are vapor-deposited onto flexible substrates made of thin stainless steel sheets of grades 1.4510 or 1.4401, are also a proven solution for metal roofs made of stainless steel. Due to their low manufacturing costs, these low-maintenance amorphous silicon cells are most commonly used in commercial, large-scale solar farms.

Frames and mountings critical success factors

Against the backdrop of an expected durability of 30 years or more, the demands on the metal components of solar panels are also high. Widely used are rails and mounts made of aluminum. However, their relatively low tensile strength makes them more recommended for low wind loads. They also require surface treatment for greater corrosion resistance. Stainless steel frame profiles ensure the required robustness and durability through superior strength and corrosion resistance, even under demanding environmental conditions.

The good forming, processing and welding properties also speak in favor of using stainless steels for frames and support structures. Weather-resistant, multi-adjustable roof hooks made of stainless steel grade 1.4016 are also the first choice for mounting the collectors. The same applies to inverters with a stainless steel housing: they convert the solar power generated as direct current into usable alternating current.

Storage for greater efficiency

The storage of solar heat that is not used immediately or of excess heat significantly increases the efficiency of a PV system. In this way, the electricity from the photovoltaic system can be used even when the sun is not shining. The higher the electricity consumption, the faster a PV system pays for itself. According to the German Federal Environment Agency, around twelve percent of a household's total energy consumption is used to heat hot water.

Today, every second PV system is already installed with an electricity storage system. This means that surplus electricity that is not directly consumed can be temporarily stored and used during hours when the sun is not shining - an economically much more attractive solution than feeding it into the public power grid for a small fee. The rule of thumb for an average four-person household is a usable storage capacity of four to six kilowatt hours. In the private sector, battery storage systems based on lithium-ion cells are therefore gaining in importance. Among the most efficient battery storage systems on the market are lithium-iron-phosphate (LFP) storage systems with a housing made of high-quality stainless steel, which are characterized by fast-charging capability, safety and robustness.

Liquid salt storage in power plants

In commercial solar power plants - parabolic trough plants or solar towers - stainless steel liquid salt storage tanks with diameters of up to 50 meters are common practice. Parabolic trough power plants use trough-shaped mirrors to focus incident sunlight onto an absorber tube through which thermal oil flows in their focal line. The solar energy concentrated in the tube made of high-temperature stainless steel grades 1.4959 or 1.4541 is converted into heat and stored in liquid salt tanks. In so-called Concentrated Solar Power (CSP) solar tower power plants, liquid salt is both a heat-transporting working medium and a storage medium. CSP solar towers are surrounded by a large array of mirrors (heliostats) that automatically track the position of the sun. These mirrors reflect sunlight onto a central receiver at the top of the tower, through which liquid salt flows. For support structures and high-strength, theft-resistant locking screws, stainless steel of grades 1.4301 or 1.4303 is the proven standard, which has also been demonstrating its performance for years in support systems for ground-mounted systems.

The hot tanks used to store the liquid salt are exposed to very high thermal, mechanical and corrosive loads in both types of solar power plant. Made of 1.4961 grade stainless steel with a plate thickness of 60 millimeters, they reliably ensure the required process reliability.

Sustainability right down the line

The use of renewable resources is the order of the day and solar energy is one of the most important energy sources for this. In addition to forward-looking technologies for its generation and use, the durability and reliability of solar systems are decisive success factors for exploiting the potential of the solar energy source even more. The origin of the system components and the life cycle assessment of the materials used also contribute to the sustainability balance of the systems. Stainless steel makes an important contribution to their positive eco-balance: Decades of corrosion and weathering resistance, low maintenance requirements and almost 100% recyclability at the end of the product life without any loss of quality make stainless steels a reliable companion for the future of solar energy.