Overview of Concentrating Solar Power for Electricity Production, with Emphasis on Steam Turbine Aspects

The European Union targets a reduction of greenhouse gas emissions by 20% of renewable energy sources in the EU energy mix by 2020. New generations of technologies have to be developed through breakthroughs in research if the vision is to be met. The deregulation of the electric power market has opened the market to investors seeking profits and growth via the installation of additional capacity. This literature review report is a introductory step towards a more detailed and targeted work for a system concept evaluation, analysis and optimization of solar turbines in concentrating solar thermal power production. Concentrating Solar Power (CSP) has grown in recent years to be the largest bulk producer of solar electricity in the world and every square meter of a CSP field that produces 400 - 500 kWh of electricity per year, saves 0,45 ton of CO 2and contributes to a 0,1 ton reduction of fossil fuels use annually. The CSP is divided in two concepts and three main technologies: Linear– and point focus concept; Parabolic trough -, Power Tower- and dish- Stirling engine technologies. Whereas the Power Tower technology has the best gross efficiency, the parabolic trough has the advantage to be the most mature technology, being in grid connected power plant operation for at least two decades. Dish-Stirling engine systems have the highest net efficiency but suffer from the sensibility to insolation fluctuations and lack of storage. There are various thermal storage medias and storage concepts to help achieving the best thermal characteristics for charging and discharging to the CSP technology in question. The dispatch time ranges from 0,5 hour for maintenance and service up to 16-24 hours thermal energy dispatchability. The most recently commercialized thermal storage media is phase change material, i.e. salt mixtures with good thermal characteristics. Research for other kind of media suitable for CSP-technology is performed continuously. The operation modes of a CSP plant depends on the choice of electricity production; dispatch electricity during day-time hours or prolonging the electricity production beyond sunset. The Organic Rankine Cycle (ORC) is another CSP concept using organic thermal fluid instead of steam to the turbine, at a lower temperature and pressure range. The power output is of range 1–5 MW for electricity production. The ORC ability to use simpler components and the possibility to deployment in rural areas as well as on limited space in urban areas, make it more economical feasible. The organic fluid is for example n-pentane and toluene. The net efficiency for ORC-plants has been proven to achieve 30% under optimal conditions, comparing to the Rankine cycle practical efficiency of 36-40 %. The experiences from CSP plant operation have raised a demand for improvement of the steam turbine’s work performance. The steam turbine has until recently being designed to be in continuous work. The steam turbine operating in solar thermal plants should have the capability to start up directly and go from idle to rated condition in a matter of minutes. The CSP-plant start-ups per year is higher where the cycling thermal stresses can cause material fatigue leading to shorter life time. Efficient collector, storage design and optimal turbine operation should reduce the number of start-ups and shutdowns per year and the transient loads on the steam turbine.QC 2012021