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The Polyphem history by KAEFER

Themis_Solar tower-2

All you ever wanted to know about the origins of Polyphem and the motivation behind the project told by our partner KAEFER (Germany). 

Since the foundation of the DESERTEC Industrial Initiative (DII) in 2009, Concentrated Solar Power (CSP) has been in integral part of DESERTEC’s revolutionary energy concept of supplying North Africa and Europe with renewable energies. As an associated partner of DESERTEC, KAEFER’s contribution was initially limited to the financial support and to the development of insulation solutions that are required for common CSP technologies such as parabolic trough and solar tower plants. Both systems are based on steam turbine technology (Clausius-Rankine-Cycle) which has been developed and used since more than 100 years in coal-fired power plants.

To make this process simpler by eliminating the steam process (no steam condenser, no high-pressure cycle, no cooling water, etc.) and therefore more economical, KAEFER started in 2009 to investigate the usage of gas turbines (Brayton-Cycle) instead of steam turbines. Within the gas turbine process, ambient air is suck to the turbine, compressed, heated and finally transferred to mechanical (electrical) energy. In addition, gas turbines offer higher efficiency than steam turbines because they operate at a higher temperature.

At the beginning of these activities KAEFER faced three challenges. On the one hand, commercially available micro gas turbines have been designed for aviation applications (light weight and high performance) and therefore are extremely costly and cause high maintenance. On the other hand, available turbines do usually have an integrated combustion chamber (inside turbine housing) whereas this CSP application requires an external connection to the heat source (solar receiver). To address these problems KAEFER has built up knowledge in turbine processes and finally in modifying turbine components to make them suitable for CSP applications. An important milestone was the proof of concept of a solar driven gas turbine that has been demonstrated in collaboration with CNRS and their receiver technology at Themis solar tower. For this demonstration, components from turbo chargers have been modified and used. Due to the higher turbine inlet temperatures and other hydraulic properties of the working medium (air), the requirements on the solar receiver are now also increasing, which is currently being researched within the Polyphem project.

KAEFER has used their involvement in worldwide projects to continuously explore global human needs for electric energy. Consequently, at an early stage the strategic orientation at KAEFER was focused on small-scale units that should ensure easy access to electricity and help people in remote areas to be independent from large-scale energy providers. For this reason, the targeted capacity of Polyphem (50 – 100 kW) follows the needs of small-scale consumers (municipalities, hotels, small businesses/ farms) in sunny regions, that have, besides their need of electricity also demand for drinking water and cooling power (air conditioning). Besides applications for single users, Polyphem can also be customized for requirements of decentralized micro-grids.

The base unit of Polyphem are a heliostat field, the solar receiver and the gas turbine, which generates electrical energy but also provides (clean) exhaust heat (about 500°C). This heat can be stored in a thermal energy storage (TES) and/or used for other applications (heating, cooling, steam generation, desalination). The overall setup can easily be tailored to the respective application. A representative example is a hotel that needs electrical power and air conditioning at daytime and some electrical energy for night time. The energy storage for night time is based on thermal heat that is transferred to electricity by using proven ORC technology (commercially available). If needed, thermal heat can also be used for water treatment by using commercially available units.

To match with these requirements, in advance to the H2020 project Polyphem, KAEFER has acquired a micro gas turbine from the aviation sector with relevant capacity and external combustion chamber to check its suitability for CSP application. In course of these investigations extensive tests have been carried out to find out limiting factors such as pressure losses within the solar receiver/heat recovery system and general performance data. The turbine to be finally used for Polyphem will be composed by state-of-the-art components but with similar performance data. The choice of these components considers the results of above investigations.

The name Polyphem was coined by Andreas Pöppinghaus, Head of Corporate Competence Centers, who looks back to many years of working with gas turbines and who initiated this program at KAEFER. The Polyphemus is a giant Cyclops in the Greek mythology that reminds, due to its one-eyed head, to solar towers that carry the solar receiver.

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