[home] [Personal Program] [Help]

---

Session: Poster session & Sponsor Exhibition
Session starts: Monday 07 October, 14:00

---

Remi Daccord (EXOES)
Vincent Rieu (EXOES)

Abstract:
The MICROSOL project, initiated by Schneider Electric, aims at developing a 10kW solar power plant for rural electrification in line with projects led by University of Liege [1], EPFL [2] and the STG NGO [3]. One of the two solutions chosen by Schneider Electric is based on two French companies’ know-how: Exosun and Exoès. Their system is based on parabolic trough concentrators, a pressurized water storage, a R245fa Rankine cycle, a scroll turbine, a dry cooler and a recycling water module. The role of Exoès is to convert the thermal power available from the solar plant in electricity, then converted by Schneider Electric’s power electronics. This paper relates the design of the power plant optimizing components, followed by test results and control optimization. GENERAL OVERVIEW The power plant specifications require 24h electricity production: 10kW during the day and 3kW in the night. The source of energy is water at 180°C - 16bars produced by a 600 m² field of parabolic troughs and stored in a 20m3 water tank. The heat is transferred through plate heat exchangers to a R245fa Rankine cycle to produce vapor at 10 to 30 bars. The expander are two 325cc and 108cc scroll turbines operating a volumetric expansion ratio of less than 3. A cold loop condensates the vapor and evacuate the waste heat through a dry cooler. The power output of the plant is controlled by the power demand on the grid. According to the available hot and cold temperatures, the feed pumps must quickly adjust the inlet pressure to reach the power output required while a super capacitor and a few batteries instantly supply the difference. Thanks to a typical load curve and weather files coupled to dynamic solar modeling, the Rankine cycle has been designed to produce 10kW during the day with high outside temperature and high hot source temperature (50% of operating time). During the night, lower temperatures do not enable the expander to produce more than only a third of its maximal load, matching the required 3kW (25% of operating time).The cycle won’t be able to produce the required power when it is too hot or if the hot source is too cold. A load shedding system is thus foreseen. COMPONENTS OPTIMIZATION We chose components providing the best efficiency. All auxiliaries have been designed to reduce their consumptions. Concerning the cold loop, only brushless motors drive pumps and fans. It enables us to reach a higher efficiency than conventional asynchronous motors over a wide range of speeds. On top of that, the dry cooler is large enough to cool down the system even with an extreme 45°C ambient temperature and it has few pressure losses. So that fans run slowly most of time avoiding cubic progression of the consumption. We swapped one turbine working 24h/24 for two parallel turbines, each having its own reserved power range. In both cases, the same 325cc turbine runs during the day so that the power plant can produce 10kWe. During the night, we chose to stop it and start a smaller 108cc turbine to have better efficiency avoiding to empty the storage quickly. This study concludes that this expander optimization leads to the characteristics below that are far more interesting to reach competitive electricity costs of the power plant. TEST AND CONTROL After a year of modeling and prototyping, lab tests by Exoès began in early 2013. Expander isentropic efficiency and filling factor can be compared to the state of the art. A cycle efficiency of the power plant can be pointed out. In this paper, we described both the difficulties we faced to start and run the Rankine cycle and the different ways to reach better performance of the power plant based on the experiment. This project is pursued by field tests in mid 2013 near Marseilles-France that are conducted by CEA (French atomic energy committee). A second test phase will then take place in Africa in 2015. LITERATURE [1] Design and experimental investigation of a small-scale organic Rankine cycle using a scroll expander, Sébastien DECLAYE, Sylvain QUOILIN, Vincent LEMORT, The 20th International Compressor Engineering Conference, Purdue, 2010 [2] Integration and optimization of thermoeconomic & environomics hybrid solar thermal power plants, thesis, Malik KANE, EPFL, 2002 [3] Solar Turbine Group (STG)’ NGO program in Lesotho, 2004