What is the purpose of the steam turbine

Steel innovations open up new applications for turbo machines

A micro steam turbine uses process steam and thus enables resource-saving and cost-effective power generation. And a newly developed turbine generator unit aims to increase the overall efficiency of commercial vehicle drives.

Many industrial processes require steam, which is often generated at high cost and with a large amount of energy. The Dortmund-based company Turbonik, a spin-off from the Fraunhofer Institute for Environmental, Safety and Energy Technology (Umsicht) in Oberhausen, founded in 2017, was awarded the 2018 Steel Innovation Prize by the German Steel Federation in the category for a new type of micro steam turbine in mid-June "Products made of steel" awarded: According to the developers, the turbine uses energy efficiently, saves costs and reduces CO2-Emissions sustainable. The micro steam turbine was developed in order to utilize existing potential in steam generation processes more efficiently. Previously unused thermal energy can be easily converted into electrical energy by combining the steam generation process and the turbine.

2017 | OriginalPaper | Book chapter

Fundamentals of fluid flow in turbo machines

Turbomachinery includes rotating machines with accelerated flow: gas turbines, steam turbines, water turbines, wind turbines and machines with delayed flow: turbo compressors, centrifugal pumps, fans, turbochargers ...

The use of extremely high-alloy steels differentiates the micro steam turbine from conventional steam turbine technology, emphasized the jurors of the steel innovation award. This means that there is no need for ecologically disadvantageous oil lubrication. Instead, water is used as a lubricant. And the direct coupling of turbine and generator in combination with an optimized impeller design enables economical operation even with small systems.

According to DIN EN 10027-1, a distinction is only made between unalloyed and alloyed steels. In practice, alloyed steels are still often divided into low-alloy and high-alloyed steels. This definition only serves the purpose of a simpler naming and does not pursue the intention to define the term alloy steel. Low alloy steels: The content of any alloying element does not exceed 5 percent. High alloy steels: The content of an element is at least 5 percent. "Material science, page 228.

Micro steam turbine in practical operation

The absence of a gearbox and the use of very high speeds have a significant effect on the size. Compared to the market standard, the diameter of the impeller is over 50 percent and the weight even 90 percent less. The ecologically sustainable generation of electricity in the small power range up to 300 kWel is economically feasible for the first time with this micro steam turbine. The developers see typical areas of application in breweries, in food production, paper and pulp processing, the chemical industry, in hospitals, in textile cleaning or at energy suppliers.

The first micro steam turbine is in use as part of a pilot project at Energieversorgung Oberhausen AG (evo), where it is used for district heating degassing and replaces a pressure reducing valve in the steam system. The turbine is driven by the steam, which regulates the steam pressure and generates electricity at the same time. The turbine produces 300,000 kWh of electricity annually, which corresponds to the annual consumption of around 60 to 75 four-person households. Energy costs are significantly reduced by the resource-saving generation of electricity and around 90 tons of carbon dioxide are saved per year.

More efficient truck engines

The Institute for Turbomachinery and Fluid Dynamics at Leibniz University Hannover was awarded the special prize "Climate Protection and Resource Efficiency" of the 2018 Steel Innovation Prize for a newly developed turbine generator unit. With the expected regulation of CO2Limit values ​​in the commercial vehicle sector are growing pressure on manufacturers to further advance the electrification of vehicles and to increase the efficiency of conventional drives.

The turbine generator unit developed by the institute and manufactured as a prototype makes it possible to use a technology that was previously mainly used in large-scale systems in trucks. A promising approach to increasing the overall efficiency of conventional commercial propulsion systems is to use the residual heat contained in the exhaust gas by means of a downstream thermodynamic cycle. The Organic Rankine Cycle, or ORC for short, named after the physicist William Rankine, is particularly suitable. During the process, a liquid, organic working medium is pumped into a heat exchanger under increased pressure and evaporated using waste heat. In an expansion machine, the steam is expanded and potential energy is converted into mechanical work. The steam then cools down in a condenser to such an extent that it liquefies again. The turbine generator unit developed in Hanover and manufactured as a prototype makes it possible to use this technology, which has so far mainly been used in large-scale systems, in trucks. In order to achieve a low weight with a compact design, the expansion part of the assembly was designed as a single-stage, axial impulse turbine.

Improved overall efficiency

The generator, which is directly coupled to the turbine stage, supplies electrical energy for ancillary units in the vehicles. An ethanol-water mixture is used as the working medium, which has a maximum temperature of around 260 ° C at a pressure of 40 bar when entering the turbine. In the single-stage turbine, this pressure leads to a supersonic flow through the turbine stage with rotor speeds of up to 110,000 rpm-1. The materials used are therefore subject to the highest demands. The stator of the turbine consists of the rustproof austenitic stainless steel grade 1.4305, which ensures the required mechanical reliability of the pressure-loaded component. A soft magnetic tempered steel grade 1.6582 is used for the rotor shaft, which meets high requirements in terms of strength and toughness. Bench tests prove the efficiency of the turbine generator unit. The turbine achieved a peak output of 7.6 kW and an efficiency of 57 percent. This corresponds to a potential saving in fuel and CO2Emissions of 3 percent. According to the University of Hanover, further savings are achieved in higher load ranges, for which numerical fluid mechanics calculations predict a turbine output of up to 17 kW. In addition, the new assembly has the potential to significantly increase the overall efficiency of conventional commercial vehicle drives and thus make an important contribution to climate protection and more efficient use of resources.